Thanks for all the input everyone!
@ColinTaylor - I am using the built-in WiFi adapter on my motherboard which is
Gigabyte Aorus Z370.
I gained about 40-50mbit by moving the antenna off the top of the desktop tower. I read that USB 3 can cause interference and it was sitting near some USB 3 ports at the back of the tower. So I am getting around 110mbit now which is better but not great. My laptop gets poor transfer speed in the same room. If I put the laptop just outside the doorway (which is about 6-7ft away) then it can get 250mbit/s. I am starting to think maybe a new antenna/adapter with a long cable routed to the top of the doorway might solve/improve things enough.
The only other room I need to boost signal in is a bedroom, and doing the laptop test in there showed speeds of about 120-150mbit/s, which is better then the office but I can't really solve that with antennas. Maybe I can mount the router that is downstairs onto the wall which would raise it up maybe 3-4ft - I'll have to test and see if its worth making that change.