I'm not aware of USB interfering with anything 2.4Ghz. If you're on 5Ghz I've not seen USB2/3 coex be a problem. Unless your dongle vendor did a bad job, USB dongles should be designed to minimize the impact of this interference because all they do is WiFi + USB (i.e. shouldn't matter if you are on 5 or 2.4). That being said, it's a simple thing to try so why not?

Does "flaky" here mean connection going in & out vs inconsistent & bad speed?

Assuming we're talking about speed rather than the connection itself, two walls can be a challenge if you're on 5ghz depending on the quality of your router & dongle. Do you have any old devices on your network? Your WiFi router will run at the speed of the slowest client. Can't recall if camping is a problem - I'm less clear about the details here, but it should be. If your 802.11AC dongle is on the same frequency as 802.11N, performance will be degraded off the bat so consider moving those devices onto a different frequency if you have a dual-band router (e.g. wireless printers or older wireless TVs can be causes).

If the connection itself is flaky, check if the RSSI is really bad. Anything >= -70 dBm is good & anything <= -80 dBm is pretty bad. Note the negative sign. -90 is worse than -80. If you're on 2.4Ghz you could also be having issues from poorly shielded electronics (e.g. old microwaves) or cordless phones.

Hope this helps!