@duckweed @linuxphones There are numerous Linux versions for smartphones (Android itself is linux-based). I use /e/ OS for privacy reasons. It was pre-installed on my phone, so I can't talk from experience about installing it, but if you'd be interested in looking at the process, it is different for each phone and they have a list here: https://doc.e.foundation/devices
/e/ OS works well for me. LineageOS also has a list: https://wiki.lineageos.org/devices/ (But I don't know anyone who uses it).

@duckweed @linuxphones So I think, about having to be an advanced user - it highly depends on your phone. Installing is far easier for some than for others.
About #security: There is an ongoing debate about it. /e/OS is probably not the most secure system in existence, but I think it is at least as secure as Android. The criticism probably comes from specialized circles - they developed some very secure systems, but they are often only available for few phones, like this: https://en.wikipedia.org/wiki/GrapheneOS