Fast download, inconsistent upload

I live in a ~2000 sqft house with a basement, a ground floor, and a second floor. I have a Pro 7 on each floor w/ wireless backhaul, and the fiber ONT is in the basement.

Speeds in the basement are consistently fast, say 900 up/900 down (Mbps). On the main floor, I also get reasonable speeds, approximately 600/900. On the third floor, downloads are consistently decent (~350-500), but uploads are either decent (~400 Mbps) or very slow (~4 Mbps). For example, I just got the following results on the second floor:

404/381 (bdrm 1)

494/315 (office)

363/4 (bdrm 2)

442/3 (office)

398/4 (bdrm 1)

The disparity between download and upload performance is surprising to me. Any theories about what could be going on or how to debug the problem more effectively?