Any omnidirectional antenna should be fine. You'll want to either get one for each antenna connection or find one that has the same number of connectors as your card.
Generally, longer antennas mean more sensitive reception and probably better transmission.
What monitor? Sounds like it's leaking EM radiation. You could probably just shield it with some aluminum foil.
I'd also check the power cable, since power circuits are usually where EM leaks originate (usually being higher wattage and much dirtier). I'd be suspicious of the transformer on the cable, if there is one.
The monitor's on the other side of the desk. It could be the issue but it doesn't affect my phone or laptop which are closer to the monitor. I will mess around with aluminum foil because that sounds wonderfully janky.
Edit: foil around cable and GPU io fixed it. It is up to about 3/4 of speed with monitor unplugged, which is a massive improvement and perfectly usable. Thanks so much for the suggestion.
Damn that's old school. Haven't seen those since the 00's.