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 think, they are not quite there yet for the moment and the average user (but I am by no means a dev. or fluent in code).
However, I can really recommend you to look at alternative Android OS'es. Here it depends on what you want.
For Privacy, the best one is GrapheneOS. You can have all google stuff (apps and google services) running sandboxed. And you can create multiple users for different purposes: like one for your daily use, with no Gapps, and another one 1/