Well I guess I didn't really break it. A KDE update broke it. After updating I rebooted, and then when I tried to log in, the screen went black and got stuck like that.
Anyway, I read on the forums that the fixed involved adding a parameter to some line in the kernel options, which I had no clue how to do. I also didn't know I could enter the terminal from a frozen screen. So I tried the grub menu. But I didn't know what I was doing and was scared to mess things up, and for some reason I thought the answer was in the UEFI screen.
Now I knew that I was treading in dangerous waters, so I was trying not to touch anything while poking around the menus trying to figure out where I needed to go. But apparently I touched something I wasn't supposed to, cause my computer tried booting from the spare SSD, which isn't mounted yet and don't know how to decrypt it. So I got stuck for a while, tried the grub rescue in the command line because it was the only option I seemed to have, didn't understand it, panicked for a while, and eventually found out I could press f2 on startup to go straight to the UEFI screen. So then I went back to the menu where I messed things up and made sure to click on the correct disk.
So I was quite relieved when I was able to decrypt it and it brought me back to the Endeavour grub menu (the purple screen), and then booted up as it was supposed to. I tried logging in again and it still froze, but at this point I had learned I could press some hotkeys to get to the terminal. So I went in there and followed some instructions I found, ultimately only really learning what the problem wasn't. It turns out the parameter I was supposed to add to fix the issue was already there!
So I found out how to revert kde desktop and workspace to a previous version from the cache, and I did that, but when I rebooted and tried logging in again it still froze.
Luckily I had previously made a guest account so I logged in there and it worked. So then I learned that that means the issue was in the user-level configurations.
So I followed some more instructions to back up my KDE configs, moved the existing ones to somewhere else, then killed and restarted plasmashell to create new default config files.
And then I tried logging in, and it worked! This was an hours-long process, so it definitely felt good to have a working system again.
Luckily most of my settings and my favorited items in the app launcher were still intact. I hadn't moved my global shortcuts config file either, so my keybindings were preserved. The only things missing were my pinned icons on the app manager toolbar at the bottom of the screen.
So I went into my backup file for the plasma appletsrc configs, and I found the line that listed the apps I had pinned, and I copied it and used nano to paste into the current version in same place it would have been.
So even though it was tragic and frustrating and a bit gut-wrenching at times, I learned a LOT today. I gained some familiarity with grub, UEFI, terminal, basic shell commands, restoring previous versions of software from the cache, logging and troubleshooting, backups, configurations, and the basic system architectures, and the anatomy of the KDE environment.
I'm still no power user, and I still have a lot to learn, but I came a long way in just one day. Now, I'm tired.
There's lots more to set up tomorrow, but at least walking into it I won't feel so lost!
Thanks! After browsing the forums and not finding the precise solution to my particular problem (and the solutions I found ended up not being right for my situation), and also realizing that the forums assume you know how to do basic things like access your terminal from a frozen desktop, edit the kernel options, etc.; and after despairing when my computer was trying to boot from the wrong SSD because I touched the wrong thing in UEFI trying to find said kernel options, and I couldn't figure out how to get back to UEFI to fix it, I ended up using Lumo to walk me through the process. It feels like a dirty secret though, cause I know how people feel about LLMs and I'm aware of some of the issues, but I made sure to double-check everything it recommended.
Details:
At first it tried to wayyy over-engineer a solution, but after some more prompting it told me that I could just press f2 at startup to get back to UEFI to boot from the correct drive. And then after that worked, it helped me get to my terminal from my frozen desktop. Then it walked me through the steps that I found on the forum, and when that didn't work it helped me troubleshoot, told me how to view my system logs and look for errors, and explained how to rollback my version of plasma from my system cache.When that still didn't work, and I tried successfully to log in to my guest account, it told me that it means the problem is in my main user configs. So it told me how to back those up and move them, and how to restart the plasma shell to create new defaults. And that's what finally worked!
Then I logged in and the only thing missing were my toolbar icons, so it told me where to find them in my config backups, and how to paste them into the correct line of my current config files, and reset the plasma shell again. It worked like a charm.
I wouldn't give it autonomous control of my system, cause the first "solution" it proposed was to unmount and remount the drives, actually physically removing them and resetting them, wipe everything and repartition/re-encrypt, then reinstall from a live USB. Like, fuck that. All I had to do was press f2 to get back to UEFI...