With arch, I’d recommend just jumping in the deep and and installing it, looking things up on the wiki when something goes wrong. Just do it in a virtual machine or on an old laptop or something to start with. I broke my previous distro (mint) by doing some really stupid stuff, and I decided to just go fuck it and install arch. I don’t think I’ve ever learned as much about linux as that weekend.
I suppose that I was already pretty comfortable with the terminal since I was studying computer science at the time so I had a lot of reason to use it. I think the best way to use the terminal is to force yourself to use it, programming is great for this but you can try e.g. sorting your photos or mp3 files or something as practice too.
I’ve been wanting a phone with a keyboard for a long time, I don’t think I’ll get this one (I specifically got a fairphone in part so I wouldn’t have to think about buying a new phone for a couple of years) but I think that this could be really nice for some specific usecases that are underserved today.