People, it is nice to hear that the installer works on your machines and VMs, but that does not really help me with my problem.
Delete the linux filesystem (other than the ESP, which I’d make sure was clear/cleaned), and allow the installer to create them during your next attempt.
The installer has done that anyways, just the ESP is still full of stuff, but I guess I’ll delete that, too.
If that fails, try installing onto another disk (ie the referenced external), even if you don’t actually want to run that way….just to prove it works to your satisfaction.
Well, I’m not livin in a computer store so I don’t really have that many spare hard drives lying around, but OK.
Install plain arch to the partitions if it fails (using archinstall), and add the cachy optimized repos, update, and pretty much good to go.
Before I do that, I’ll switch distro. Really, I just want a working system, not another hobby 
Just nuke the drive.
Not an option. I need that Windows because nobody is able to run Boss Tone Studio on Linux and I really need that.