Previous blogs have mentioned K-9 is just being rebranded as Thunderbird for Android once the Android app is closer in features to the desktop release. The iOS release will be new entirely as I don't know of an existing iOS email app they can rebrand.
Have to also add to the voices recommending Debian stable. I've used it now for ten straight years after I stopped distro-hopping for my servers and desktop, and I cannot imagine using another distro. It's incredibly stable, but the best part of Debian is the absolutely expansive repositories that even the Arch User Repository can't beat. Very rarely do I ever need to use Flatpak (ugh) for packages, or look to add in new external repositories.
It's listed as "medium difficulty" due to newer hardware in the laptop not being fully compatible with the kernel shipped in that old version of Ubuntu. I believe 22.04 is compatible out of the box.
I had to use a Debian sid nightly installer to set up Debian on my laptop, no big deal for me but for someone new to Linux I can see why that might be off-putting.
We used to run firewalls running Fedora at work, works fine. Issue is you're only getting 6 months of updates, best to look at Rocky Linux for something that doesn't change much if you do anything beyond a single program.