Have you considered bringing an AP closer to the client? Especially on 5GHz a good line-of-sight / line-of-reflection is very helpful, and some extra 10-20dB of signal should compensate for a lot of interference.

OFDM MU-MIMO AX 20000+ 1000$ routers with 20 antennas work better, but in an inconvenient location, when the noise floor is high, it has no chance to compete with a decent 2-antenna ac (or even n) AP with good signal propagation (same room vs. behind a wall somewhere).

And it would be helpful if you can agree with your neighbors to use narrow channels (less competition and unintended interference), decrease radiation power (less distortion => better signal) and if supported, increase minimum bitrate (faster communication => less jamming). Also, lower transmission power and faster SSID broadcasts help induce roaming, for when you have multiple APs.

Building a performant wifi network is a serious pain. Building material, channel congestion, network load, AP placement, other sources of RFI, etc all cause issues.

Your best bet is look for APs that support 802.11r or v (also check that clients support it though most modern cards do) which allows seamless roaming between APs then replace the existing AP with 2 or 3 APs. If you want a cheaper solution you can try for a DIY site survey to try to find the best placement and channel for the existing AP.

There are client priority features in wifi, but your device may not support them. If her antenna has line of sight to the BS then it's not likely to be interference but something else, like traffic congestion.

I'm not sure what hardware/chipset you're using, but if it's consumer grade, that might be a big part of the problem. Competently executed prosumer/soho like the amplifi/unifi line or google wifi may make a huge difference.

However, ethernet may just be the best option, especially for streaming.

In my view: If the packets absolutely positively have to get there on time, ethernet is the only option.

WiFi has uses but constantly streaming large volumes of data without hiccups isn't one of them.

With wifi, you need to make accommodations as well. It's not just Magic Internet Waves. I've had no problem video conferencing on wifi, but I make sure to have reasonable line of sight to my AP. I also chose a model with a simplistic and streamlined design so that it didn't look awful placed prominently in my space.

> well over 100 APs..

With that many neighbors I am not sure if there is much that can be done. You can take steps on your network, but getting everyone else to cooperate is a different story. Even if they are willing, how many would be able to manage their channel allocations and transmit power?