A removable physical or electronic SIM on a system that has full control of inbound or outbound traffic (linux phone) would still be a whole lot better than nothing. Imagine having a switch to reliably sever any heartbeat signals between the tower and the device at any time.

Yup, the baseband modem does what it’s firmware tells it to, and that’s entirely independent from the phone’s software. And open baseband modems to my knowledge don’t exist.

If the spyware/tracking started and ended at the cell tower it would be a good start. I'm not sure the sensor data would be sent to the tower either. It would just be a general area.

A Linux phone could theoretically use other networks. You could pipe traffic through I2P or bounce it around multiple network types with reticulum. It’s actually theoretically possible to make a community mesh that doesn’t need cellular at all. I don’t NEED to carry the entire internet with me everywhere. I can carry a device with a cache of stuff I need but for everything else I can just connect to some sort of network to fetch it when I actually need it on demand.

A Linux phone would let you do that. You can explore that possibility. Android and IPhone will never allow that because latency is shot on the alternative networks and they aren’t expensive enough to make a profit off of.