I've been reading some articles about mobile Linux and many of them state one must be an "advanced user" or that the software isn't secure. How true is this?
I already use Linux on my laptop, but I'm not a software developer or anything like that. Would I be able to slap a new OS onto an old Android phone and be on my way, or would I run into problems?
I can also recommend you CalyxOS, which runs google services (but doesn't have to).
If you care for longevity, LineageOS is the way. They support phones way past their official update cycle - it comes at the cost of some security once the security patches of your phone runs out.
Sorry if I went a bit overkill on the answer :BlobhajBlanketBlue: !