Problem solved! In my situation anyway - and this was an unexpected mechanical thing. I have a MacBook Pro 16" with M1 Pro chip. It has 1Tb inside and so I have an external larger Thunderbolt SSD for video connected by an OWC Thunderbolt 4 cable. That all works fine.
To make cable routing tidier I bought a nice little metal 90deg. USB-C connector to direct the T4 cable straight backwards rather than sticking out to the left. I half assumed it might not work at all on fussy Thunderbolt, but in terms of data and power pass-through it worked just fine.
But then the WiFi problems started as reported elsewhere: lousy speeds, dropout, and computer falling off the network. I spent much of the day rebooting routers etc. as we do, assuming it was a software/IP issue somewhere. But then SpeedTest showed me I was getting good WiFi and full speed with nothing connected to the laptop's ports but things fell off a cliff with the SSD plugged in. And then... eliminating the tiny adapter and just using the OWC cable between the laptop and the drive solved things entirely.
Such a thing hadn't occurred to me and I don't know what leaky radiation is interfering with the Wi-Fi signal from such a tiny physical element, but clearly it is. So... if you have WiFi problems, it might be worth looking at what's plugged in and what other interference there might be nearby.
And if anyone can explain the science, that would be of interest.