TopDog, I should have responded to you specifically as well...
My channel and power were locked.
How congested? I'm not sure how to quantify that. In terms of other SSIDs on the same channels, not a lot. I live in a residential neighborhood with quarter acre lots where the neighbors' SSIDs don't bleed too far into my own house, and there are few visible SSIDs. As stated, I am using 1,6, and 11 amongst all my APs, and each of them exhibited the same issue.
I disabled ANI, applied the hardware ack (ath9k nohwcrypt=1) change, as well as disabling low ACK disconnects with no appreciable improvement. I also tried using the ath10k ct-htt drivers, though I don't think those apply to the 2.4Ghz radio.
If I could, I'd love to try the ar71xx images on the A7. I suspect this is something unique to the newer ath79 drivers.