And it went on to happen in the loop: login -> white screen -> reboot.
Recovery mode + macOS reinstall = "Can't install macOS on this computer"
To my that starts to match the definition of "bricked" already, because a normal person really can't probably recover from this at that point.
Recovery mode (booted via USB drive) = "Installer is missing assets" (tried 2 USB drives, after re-downloading Catalina installer
Recovery mode (booted via USB drive with Mojave) = this is not supported anymore
Recovery mode -> Disk Utility -> First Aid -> run a FS check, turned out APFS volume is corrupted ("fsroot tree is invalid")...
Target mode (went to BestBuy to get a special "SS+" certified USB-C cable, because of course a normal cable doesn't work) connected to my macOS Catalina Mac mini -> couldn't unencrypted the APFS partition enough though the password was correct. Read every possible target mode tutorial, and there's no mention about what Catalina did to Recovery Keys/User (other than them simply not being there), but you need to pass a UUID of a recovery used, and I only had some iCloud Recovery Keys there. Tried my MacBook password, my iCloud password etc. Nothing worked. Basically I couldn't un-encrypt the drive.
Ended up just starting Recovery Mode again, mounting a separate USB disk from the Terminal, copied valuable stuff over.
Removed all APFS volumes, tried to re-install on a fresh volumes. Nothing worked. Was getting the same issues from the installer. Gave up. Packed this $4k computer and sent it back to Apple. Writing this from my Mac mini, with Catalina, happily rejecting any updates. BTW, Catalina installer is a joke. It is broken since Catalina came out, and it's not getting any better.