1. Android with unlocked bootloader and AOSP + F-Droid apps: this looks like it would work but it's really hard to find up-to date info about what manufacturers/models allow to easily unlock the bootloader, and what are the best alternatives in terms of AOSP distributions, what's the best source of information?

2. iOS: is it possible to run iOS while preventing Apple from gathering personal data? Does the above require jailbreaking? Is jailbreaking still commonplace/viable as of 2018? And even then, does iOS have an equivalent of F-Droid?

3. custom OS, such as the Librem by Purism, however these are either very expensive or yet to be available on the market. Or is there any alternative that I haven't heard of?

4. just use a dumb phone. Has anyone here tried it? How did it work for you?