Do yourself a favour and install Linux Mint Debian Edition 6. It's Debian 12 with all the good tweaks made by the Mint team..

Based on Debian, with just the right amount of user-friendly additions is MXLinux. Coming from Ubuntu, installing MX was particularly easy, the small community is very helpful and knowledgeable, and any quirks Debian might pose to a desktop user seem to have been ironed out.

One other idea regarding the external drive. It probably won't resolve the issue, but it's not too hard to try and it's simple, so I'd be inclined to give it a stab.

If your external SSD drive has a power connector separate from the data connector, which some do -- that is, it's not relying on USB or similar for all its power -- and it hasn't been powered down since you hit the problem with your Arch installation, I'd try shutting down the computer, then killing the power entirely to the drive. Then power it back up, and start the computer again.

I have, before, seen USB-attached devices that manage to get themselves into some sort of funky state. Normally, rebooting the computer resolves that, but with devices with an external source of power -- which can be the case for external drives -- I have seen devices sometimes not cleanly reset when the computer does and stay in a bonkers state.

Just asking: how long have you been using Arch and why? What qualities did you like in it?

Going from Arch to Debian is a huge leap. In my personal opinion, Debian is a great distro for servers or really really conservative desktop users, but it gets stale really fast.

Something in between both is ideal for deskop use, like Fedora, Ubuntu, Mint, etc.. The half year release schedule keeps everything modern, but stable enough.

You said in another comment, that stability is the most important aspect for you. I recommend you...

Fedora Silverblue

Why?

• Great update schedule (see above)
• Extremely stable. Fedora at it's base (already pretty reliable), immutable base (less bugs, since that's more reproducible and therefore easier to fix), also
• Atomic updates. You either apply a functioning update, or no update at all. If you update on a traditional distro and loose power, it is only applied partially and your system is borked
• You can always rollback with one click if an update isn't working as it should (e.g. screen flickering)
• Seamless updates. They just get installed in the background and when you reboot, the next image is already selected for you. I don't even notice an update and never get annoyed. I shut my PC off anyhow every few days, since booting takes just a few seconds on an NVME.
• Base can be exchanged with one command. If you run Gnome and want to switch to KDE, you rebase with one command, reboot, and everything Gnome related is gone and KDE is installed cleanly! Feels like a reinstall, but your user settings and data are all still there. You can also rebase to something from Project uBlue, which offers custom images, like a SteamDeck-clone, different kernels, Cinnamon desktop, and so on...
• Huge software repository. You (should) never install .rpm s directly to your system, you use containers. Flatpak is great, but Distrobox even more! You can access the AUR too if you want and use those apps just like natively.
• And so on

I just Google "user not in sudoers Debian" and copy everything into a terminal - after booting into the Root shell - and forget about it.