Thanks for the thoughts, I appear to have fixed it. I just went into "Settings" everywhere and turned off anything that looked remotely related. The phone spends 98% of it's time with SIMs off and IoT networks on. I'm not driven by the need for a constant stream of inane static. The phone for me is a tool. I turn a SIM on when I need to call.
I recently bought it for the 5G and tethering, but now that I cannot get anything much above 9Mbps tethered it is useless for that, so it is just another "hammer" in the toolbox.
As to my networking, I suspect you assume too much? Individual hardware routers are so much easier and reliable than anything Virtual. Been there done that came back to the simple hardware. If you would like a couple of managed switches for free, I have them in the garage somewhere. Experience has taught me that the latest gee-whiz is rarely the better option. Known as the KISS principle. :)
Also my IoT is all of my own protocols using Arduino and Linux PCs running my own monitoring applications (C++/Pascal/Assembler) --plus writing several .apk Apps for our phone. When I began building the IoT stuff MQTT was not yet a "thing" in IoT. I have 52 nodes purring away 24/7/365 monitoring tank water levels, barn temperatures, doors opening and closing according to sunrise/sunset.
I programmed for a living for 40+ years (built and sold 2 software businesses) so I feel qualified in making my own choices.