Which recovery should be used? The one from the first post (Required TWRP recovery: TWRP for system-as-root), or the one from download.lineageos.org (lineage-17.1-20200405-recovery-chiron.img)?
Maybe this helps someone:
I had LineageOS 16 latest nightly, TWRP 3.1.1, fw 9.*, magisk
I flashed fw_chiron_miui_MIMIX2Global_V11.0.6.0.PDEMIXM_0805fdbc02_9.0.zip in the recovery.
Everything still works.
I boot to bootloader, and try to flash latest nightly lineageOS recovery. It says flashing... and no other output. I tried waiting, once even hours, nothing. I found advice to use the original cable, to use USB 2.0 ports, didn't help. I gave up, continued using LOS16.
Next day I googled you can use the flashify app to flash a recovery, I tried it and it worked. Now I have the LOS recovery, so I try to flash 17.1, and gapps over my existing system. Doesn't boot (= got the "can't load android system your data may be corrupt" message).
I have a backup, so I just reset data and try again. Same result. Next I tried: Flashing firmware together with los and gapps. Adding magisk. wiping /data /cache and /system. All the while I'm getting the "false" error message that signature couldn't be verified (what does that mean, did it mismatch, or is it just not present? Does it mean a cryptographic signature, or a checksum?), and the transfers (using adb sideload) are stopping at 47% (also not a problem). I probably tried more things I don't remember.
Then I thought maybe it's the recovery, let's try flashing the one linked in the first post of this thread. But now I don't have a working os (for flashify), and fastboot is still not working. So again I tried many things, including a second windows pc. Nothing.
As a last ditch effort I connected it to my Arch Linux server and tried fastboot. It worked. (I flashed the same lineage-17.1-20200407-recovery-chiron.img I've been using before) I go to recovery, flash os and gapps, and it boots. After a few hours everything seems to be working fine.