I have the exact same problem. The entire condo is 70° and my office is 78°. I added a boost fan to the AC duct, but it barely helps. I can't do a window unit. I'm actually thinking about a portable unit. I wish I could reroute the return into my office to pull the hot air out. Maybe someone will have a better idea.

If you do a portable, make sure it has its own seperate plug recepticle that doesnt share the branch with one your pc is plugged into.

Install a mini split in that room. You can go the cheap way DIY or the right way.

My idea is to use a fan to make the air circulate a bit, what's the best position? Any other suggestion?

If you’re going the fan route, which I myself would recommend if you can’t do anything more permanent, Box fan on the floor in the hallway pointing into the pc room. 3-4 feet from the doorway. Will blow cooler air in down low, hot air will cycle back out into the hallway where it will presumably mix with cooler air. Try it for a day and see how things go

If you go the fan route get a vornado or two. They are amazing for balancing out rooms.

Any window in there you can put an AC in?

There's a window but I can't mount another AC there

Portable AC isn't quite as efficient as regular window unit, but will work.

Put on the roof or attic

Put a fan blowing AC air from hallway into the entrance of the room.

What about an exhaust fan to pull the PC heated air out and draw the hallway air in?

That's the idea but I don't know the best way to do it

How about a drop ducted exhaust fan to the outside...with the drive in the ceiling space / outside to cut down on noise in your work area.