Well, uhh, these requirements. But idk.

Yeah I’ve heard Arch is lightweight, but that might happen in like 5 years :D

Linux Mint is the easiest one from the list, but all of them except Solus are fine. I personally recommend Mint or Debian, Debian Sid if you want latest kernel.

No, I don’t really need it.

KDE should work fine, maybe with a bit of tweaking?

I thought about that too. If that doesn’t work out, Xfce/LXDE/LXQT it is.

Well uhhhhhhh, when my "laptop"s plug is pulled out or the power goes out, it’s dead :D. might need a battery change or an ups…

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Well, Mull is more like a “Librewolf for Android”, I (think) it is more privacy-focused than Fennec.
Mull also supports more extensions from developer options.

Check these out for Librewolf and Mull’s results. Fennec isn’t in the list (yet?)
https://privacytests.org/
https://privacytests.org/android.html