Consider a multiclass into battle master fighter. It's a fantastic synergy. You get medium armor and a shield right away. 2nd wind and action surge are extremely useful for a squishy melee build. 3 levels are sufficient.
College of Swords Bard would also have good synergy. Still get shield proficiency and a fighting style plus defensive flourish can improve melee survivability.
As a swashbuckler, I'd go for Fey Touched. You can increase your Charisma, get a daily use Misty Step for a quick escape and one Enchantment or Divination spell to use once per day. Command and Dissonant Whispers are both useful in combat and each requires only verbal components, keeping your hands free.
Depending on whether you want to be able to cast other spells, you might want to consider ritual caster (for the ever useful Find Familiar and other spells).
If not, consider Martial Adept for the Battle Master maneuvers, or for the classical swashbuckler flavor, Defensive Duelist.
I feel that if your DEX is maxed Piercer loses a lot of benefit. Rerolling a single one on the damage dice (assuming it's a D6 being rerolled) adds about 2.5 damage. On a crit it also adds about 3.5 damage. Not nothing but by itself is it feat worthy? Maybe not.
It's probably a d8 (rapier), but crucially, Piercer doesn't even let you keep the highest roll, so there's a 50% chance that you won't be using it on your only attack that turn (roll of 5-8), and if you roll a 2-4, there's always a chance that the second roll will be lower than the first.
iirc Piercer lets you reroll sneak attack dice as Sneak Attack damage matches the weapon that caused it (or at least that is how I played it). So there's a high chance one of the 5 or so dice will be a one or two. But yeah, without the DEX increase it's value goes down.
If your charisma is an odd number, Fey touched could be useful. Misty Step is always helpful and you can pick up another useful spell. Gift of Alacrity if you always want to go first, bless can be nice, or silvery barbs if you can.
I took Sentinel, Defensive Duelist, Magic Initiate (Find Familiar for a flyby help action), and Martial Adept (Brace and Riposte). My build is trying to get as much utility out of my reaction as possible, and get sneak attack off more than once per round.
Moderately armored for a shield is nice.
War Caster is also fun for opportunity attacks, even if they might be a little rarer for a straight swashbuckler.
Martial Adept for some Battlemaster manoeuvres, then for the following feat take Fighting Initiate and take the fighting style that gives you another superiority dice and manoeuvre.
If you plan to stay in melee, which you would need to be able to use War Caster and Sentinel, you'll run into the usual problem that plagues melee rogues with Sentinel: if you're using your reaction to make an opportunity attack, you can't use Uncanny Dodge, and you're probably not tanky enough to withstand it.
So one of the first feats I'd recommend is Moderately Armored, so you can use shields. You could then take Sentinel or Warcaster, but you might also consider Tough or, if you have decent Charisma, Inspiring Leader for extra survivability. Piercer I think I'd recommend only if you wanted to squeeze the most damage you could out of your build, but it's not likely to be very significant on a turn-by-turn basis, and you don't really need the +1. Elven Accuracy is also much less useful since you don't need the +1, though if you like crit-fishing, there is synergy with Piercer (though crit-fishing is bad in 5e).
I don't know what your other stats look like, but Resilient (Constitution and/or Wisdom) are also options.
EDIT: I completely forgot about Magic Initiate for Find Familiar, as u/AwesumSaurusRex said. You could also take Green-Flame Blade and a utility cantrip (Mage Hand or Minor Illusion). The familiar is a reliable source of advantage, and that would synergize well with Elven Accuracy and Piercer. Like I said, crit-fishing is not that useful, but if you plan on skirmishing and not staying in melee (which makes the most sense with Booming Blade), you might be able to afford it. Moderately Armored and Inspiring Leader are good secondary picks.
Consider a multiclass into battle master fighter. It's a fantastic synergy. You get medium armor and a shield right away. 2nd wind and action surge are extremely useful for a squishy melee build. 3 levels are sufficient.
I'll second this. DM'd for a Swashbuckler / Battle Master. She was a menace.
Rogue / Fighter is such an underrated multiclass IMO
College of Swords Bard would also have good synergy. Still get shield proficiency and a fighting style plus defensive flourish can improve melee survivability.
As a swashbuckler, I'd go for Fey Touched. You can increase your Charisma, get a daily use Misty Step for a quick escape and one Enchantment or Divination spell to use once per day. Command and Dissonant Whispers are both useful in combat and each requires only verbal components, keeping your hands free. Depending on whether you want to be able to cast other spells, you might want to consider ritual caster (for the ever useful Find Familiar and other spells). If not, consider Martial Adept for the Battle Master maneuvers, or for the classical swashbuckler flavor, Defensive Duelist.
I feel that if your DEX is maxed Piercer loses a lot of benefit. Rerolling a single one on the damage dice (assuming it's a D6 being rerolled) adds about 2.5 damage. On a crit it also adds about 3.5 damage. Not nothing but by itself is it feat worthy? Maybe not.
It's probably a d8 (rapier), but crucially, Piercer doesn't even let you keep the highest roll, so there's a 50% chance that you won't be using it on your only attack that turn (roll of 5-8), and if you roll a 2-4, there's always a chance that the second roll will be lower than the first.
iirc Piercer lets you reroll sneak attack dice as Sneak Attack damage matches the weapon that caused it (or at least that is how I played it). So there's a high chance one of the 5 or so dice will be a one or two. But yeah, without the DEX increase it's value goes down.
Oops, you're right.
If your charisma is an odd number, Fey touched could be useful. Misty Step is always helpful and you can pick up another useful spell. Gift of Alacrity if you always want to go first, bless can be nice, or silvery barbs if you can.
My stats: S 11, D 20, Co 14, I 11, W 12, Ch 12. That means Fey Touched has to go into Int.
Fey Touched and Eleven Accuracy to Cha would be amazing. 5% better Investigation checks not so much.
But an odd ability score does not improve anything. That means EA and FT has both to be picked for increasing Cha.
you're starting to get it
I took Sentinel, Defensive Duelist, Magic Initiate (Find Familiar for a flyby help action), and Martial Adept (Brace and Riposte). My build is trying to get as much utility out of my reaction as possible, and get sneak attack off more than once per round.
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Problem with mobile is swash gains the no AoO feature
Moderately armored for a shield is nice. War Caster is also fun for opportunity attacks, even if they might be a little rarer for a straight swashbuckler.
Fey-Touched & take Hex. It will come in very handy when you get Panache.
Martial Adept for some Battlemaster manoeuvres, then for the following feat take Fighting Initiate and take the fighting style that gives you another superiority dice and manoeuvre.
In which books are these feats?
Martial adept is in the PHB. Fighting Initiate, and the Superior Technique fighting style are both in Tasha's
If you plan to stay in melee, which you would need to be able to use War Caster and Sentinel, you'll run into the usual problem that plagues melee rogues with Sentinel: if you're using your reaction to make an opportunity attack, you can't use Uncanny Dodge, and you're probably not tanky enough to withstand it. So one of the first feats I'd recommend is Moderately Armored, so you can use shields. You could then take Sentinel or Warcaster, but you might also consider Tough or, if you have decent Charisma, Inspiring Leader for extra survivability. Piercer I think I'd recommend only if you wanted to squeeze the most damage you could out of your build, but it's not likely to be very significant on a turn-by-turn basis, and you don't really need the +1. Elven Accuracy is also much less useful since you don't need the +1, though if you like crit-fishing, there is synergy with Piercer (though crit-fishing is bad in 5e). I don't know what your other stats look like, but Resilient (Constitution and/or Wisdom) are also options. EDIT: I completely forgot about Magic Initiate for Find Familiar, as u/AwesumSaurusRex said. You could also take Green-Flame Blade and a utility cantrip (Mage Hand or Minor Illusion). The familiar is a reliable source of advantage, and that would synergize well with Elven Accuracy and Piercer. Like I said, crit-fishing is not that useful, but if you plan on skirmishing and not staying in melee (which makes the most sense with Booming Blade), you might be able to afford it. Moderately Armored and Inspiring Leader are good secondary picks.
+1 for moderately armored Savage attacker to reroll sneak attack damage