Idea: take the open source Beagle Bone Black, which has open source schematics, put into (probably chunkier) phone form factor, add 4g phone modem. Use containers to segregate any apps, e.g. web browser, maps, Signal/Telegram all in a separate container from each other. Would this allow for less privacy-invasion from Google etc.?
It's better to use something like i.MX 8M Quad - you may want to take a look at the Librem 5; or if you want something lower-end than that then there's also the PinePhone which is based on A64.