thanks, I actually found an easier way from this site, as fastboot is still working:
return opo to stock (without reset tamper flag)
then i flashed the
LineageOS recovery (twrp didn't work the last time, that's how the trouble started...) and then directly rebooted to recovery (as stock OS apparently removes custom recovery). so, next steps from fastboot mode: pull out usb cable, press power button to turn off, and then vol down+power to get into recovery.
Followed instructions to
manually upgrade LineageOS to latest version, installed opengapps for Android 11, arm, pico (nano version was to big, surprisingly).
Systems runs pretty smooth, i am surprised
Now I can continue using the OPO (as secondary phone, btw)