@brad it will be rad if/when the nixos mobile thing ever happens. i currently run nixos on my desktop and i like it. there are a few rather big things that frustrate me about it tbh but overall it has a lot of appeal. haven't tried guix at all yet but there are definitely some things guix does that nixos doesn't (like not force systemd) but the inverse is also true. pretty sure guix doesn't have anything like flakes, for example.
i have only used the desktop version of debian for like a few hours probably ever so i don't know much about it but i have a lot of respect for the debian project and i had a pleasant experience using it for the small amount of time i did. it's been around for ages and i like a lot of what they've done for gnu/linux/free software. i disagree with their systemd decision which didn't feel very "debian" of them but i never looked into it that much so i can't really comment on it.
i am not a fan of manjaro personally although i can see why some people like it. i used it for a few months but arch ended up being much more to my taste and it didn't break as much. i run arch on my thinkpad (x220) right now. i may end up making my desktop run arch and my thinkpad run nixos but i doubt my laziness will ever allow me to see if i like that way better or not haha.
i really hope a lot more apps &c. get optimised for mobile use quickly and that the mobile linux experience keeps improving. i think i will likely end up using postmarketos in part because it seems pretty popular and that means free support on fedi or xmpp or wherever i do my online socialising lol