Regarding the netperf behavior, I'm currently trying to disable "airtime fairness" (based on this suggestion - see also this) on master to see if that has an affect. My first attempt didn't work. It's slow going as the AP is in almost continuous use and I have limited opportunities to flash new images (even a simple reboot can be disruptive). Assuming 19.07.4 does not have airtime fairness enabled, its likely faster to try a test with that...
One other suggestion is to try a non ct ath10k firmware/driver. For this device, I've found that the ct driver firmware work well (stable and as fast as I need) and if problems come up with these, the dev is responsive.
By contrast, the non-ct ath10k firmware used by openwrt for the 9980 has not been updated in 5 years, I don't know it's origin beyond this, and I don't think the non-ct ath10k driver is supported or updated by openwrt at this point. When I've tired the non-ct driver/firmware in the past they cause issues for me. I have yet to try the non-ct ath10k driver again - its on my to-do list.
Regarding ath10k firmware (and board files) for 99x0 devices, there are other options (i.e. the stock netgear r7500v2 images which are still getting updates) but until now I didn't understand how to try those. I'll get into details on this "side show" in my next post.