twrp failed to sideload
Not sure this counts as an answer. But I prefer to summarise what I learnt from the long comment thread. Note these are lessons learnt from flashing 4 Android Phones in the last 3 months:

Check your usb-cable before starting. If it doesn't connect to the laptop where your adb is installed, get a new one!
If you install want to unofficial builds, try to have at least another alternative ready for installation. Best try to have at least one alternative stable build ready for installation.
Unofficial builds may not support ADB as @alecxs pointed out in the comments
Have an OTG cable ready! I didnt have one ready, but ordered one and left the phone in TWRP for the days until it arrived. With the OTG I managed to install without issues

If 1-4 do not work for you, then I suggest rebooting into bootloader. Thanks @Evan Carroll for the link: Is it possible to boot into a different recovery from TWRP?