Linux is the obvious choice for a non-Windows OS, but which flavor will be a forever argument. I currently run three different Linux distros on different machines (four, if you include the Steam Deck, but let's not). Hardware support for the T431s should be quite good.
Arch Linux is a good choice if you want to be on the bleeding edge. Install it and the KDE Plasma Desktop and you've got a really capable OS with good tooling and a great UI. However, it almost always requires using the terminal for maintenance tasks and the dependency resolver sometimes needs manual intervention.
Arch has a helpful community forum, and excellent wiki. I run it on a Framework 13 with 16 gigs of RAM.
Fedora is not as bleeding edge as Arch, but it's close. It does try to make everything more user-friendly and GUI-oriented. I highly recommend the KDE Plasma version, but I really can't stand Gnome so that's personal preference.
Fedora handles updates through a reboot process. It may be cleaner, but if you are averse to rebooting you might find it frustrating. Also, unfortunately they've shipped some very annoying, near showstopper bugs recently, with kdewebkit not rendering at all on AMD GPUs and most recently screen locking breaking entirely. These have both been resolved and had workarounds, but probably shouldn't have made it into production. I run Fedora on my gaming PC as I wanted something more "mainstream" than Arch, wanted to stay near the bleeding edge, and refuse to engage with the Ubuntu organization for personal reasons.
Linux Mint is pretty good, and I have it installed and running well on a 2013 Macbook Air with 4 GB of ram. Booted into a full desktop with Firefox it uses about 1.5-1.8 gigs of RAM (before you start loading up on tabs) and is very quick and responsive for such an old machine. I wouldn't personally use Mint on a workstation myself. While it is more stable, it also means that key application and kernel updates I want to use take a long time to filter in. It's very polished though.
Last, I used to use Pop!_OS on the Framework machine. Interesting, but honestly I much prefer the vanilla Plasma desktop to anything else I've tried.
AnduinOS
https://www.windowscentral.com/software-apps/a-microsoft-eng...
Thinkpads tend to be well supported
