[Redmi Note 12 4G] fruit_against_teeth:
So I finally did it and flashed LineageOS. Scroll down for the step-by-step manual.

In the end, it wasn't even all that hard once I got to the point of having OrangeFox which is basically where I was last week. I cannot recommend flashing OrangeFox again using OrangeFox after you pushed it into the recovery partition (slot B of your A/B device) using fastboot. If you do that it will automatically flash itself to the A slot as well. Once that happens and you reboot, you will be in A and if you then flash something using OrangeFox it will be flashed into the B slot - which makes no sense since this is your fallback. I didn't find an option or something to reflash the slot that you're already in, so I'm quite sure that even tho it is recommended, it's not really necessary.

I tried several times to flash PixelExperience as well as LineageOS and cleared the cache before rebooting (which is something I had forgotten to do before) but when the phone tried to boot them up the display would flicker a few times and I would end up in OrangeFox again (meaning it fell back to the recovery partition). I also didn't use preflash this time as it isn't necessary with OrangeFox and performed the ROM flash directly without the queue but I got the same flickering display and ended up back in OrangeFox.

At some point I actually watched the flash process and I realized that there was a terminal-like view (the so-called log screen, which you can manually open by pressing the small "^" sign next to the "go back" button) with notifications while OrangeFox installed the image. And on it there were status and error messages from the last attempts to flash something as well as the boot attempts. One message stood out: "Failed to mount system" or something similar. Since this was the only error message there I assumed that this had to be the issue. I once again wiped everything but got the same result. So i finally formatted the data, rebooted - and it actually worked.

So in the end the process is really simple. All you need is a SD card in your phone and a Windows PC!

0. BACKUP ALL YOUR DATA!

1. Install adb and USB drivers.

2. Download OrangeFox, extract it and put the recovery.img into the same folder as your adb and fastboot

3. Put your phone in fastboot and connect it

4. Open a terminal (cmd) and navigate to the folder where you put your fastboot

5. Push OrangeFox onto your recovery partition using the command "fastboot flash recovery recovery.img"

6. Boot into your recovery using the command "fastboot reboot recovery"

7. Once in OrangeFox, your phone will be visible in Windows again, so you can copy the ZIP-file of the ROM you want to install onto your SD card.

8. Wipe "Dalvik / ART Cache" and "Cache" (trash bin symbol in the navigation bar)

9. Go back to the file view (folder icon in navigation bar) and go to your SD card. It will be the folder "sdcard1" in "/" (root). Find the ZIP you want to flash and tap on it.

10. In the dialogue, activate the "reflash OrangeFox afterwards". You shouldn't really need it as it is in your B slot and your flashing slot A, but better safe than sorry. Swipe right to start the process.

11. Wipe cache (it's a touch button that appears after the flash finished) and reboot (another touch button directly next to it).

Chances are, the boot will fail and you end up in OrangeFox again. If that happens, format your data (trash bin in navigation bar, right register) and reboot again and it should work.

I hope this will help total newbies like me to not take 3 months to finally flash their first custom ROM.

Thanks for all your help. I'm still unsure if I want to sideload Gapps as my first interactions with LineageOS without Google Apps felt very weird. I will try to find all the apps that I need (or alternatives of them from the F-droid store) and give it a try without Google. If it bothers me too much, I will either reflash LineageOS and add Google or use PixelExperience.

I also have some laggy interactions, especially when I switch apps. But maybe I just haved to get used to it.