I can't decide between NixOS and Gentoo

So I'm in a bit of a unique situation. My Thinkpad with Arch is running since forever but on my Desktop I keep rolling Distros. I've been through Fedora (twice), Debian, Mint, OpenSUSE, Arch and Void. None of them scratched the itch like Arch on my laptop but I can't really have everything relying on the same bleeding edge distro. Currently it's Fedora but I'm gonna have to reinstall anyways so I might as well take the chance on another one.

Today I ordered a 250GB SSD strictly for the OS with my home folder being on my 1TB main drive. So I don't have to constantly reinstall steam games and shift backups around when something goes wrong. Now the kinda unique thing is I'm gonna be without internet for a month in December because I'm switching my provider. I need a distro that is really reliable and will just work without breaking. I really don't want to reinstall all my programs via my metered mobile Internet but I also want to challenge myself a little.

So my ideas were either make a super optimized Gentoo build in that month where I still have my Internet, maybe create an ISO image of it too. Or NixOS with the packages I need downloaded so I can reinstall it offline if that's possible.

I'm gonna have a Bazzite ISO if something goes wrong so I can still kill time with steam games but I'd like to try something new.