While GrapheneOS is a good alternative, it is ultimately dependent on Google not declaring an all-out war on custom ROMs (which is a possibility, however unlikely). Ultimately, the only true way to have FOSS phones is Linux (yes, Android is Linux-based, but not exactly as free as the Linux of desktops)
For individuals who want encryption, wouldn’t client-side checks (aka self hosting) be better than trusting all European services?