This is the type of thing that AI is actually good at diagnosing in my experience. Haven't had anything similar happen but seems more of a router issue upstream.

Maybe worth checking what Steam Deck's connection has configured differently given it's on the same network?

What is the constant? You have something that is unusual and that has not changed for 18 years. Is it specific to your home network?

I have not had any issues I can remember with Linux wifi for as long as I have used wifi.

Interesting; I haven't had wi-fi issues in Linux for more than a decade, but admittedly I sort of selection-bias towards laptops that are known to work fine with Linux.

You can mess around or go buy a $10 gbit USB dongle that you know works like a tplink.

Like what sort of response times for each?