I just installed Droidian a day ago. The installation process didnt worked out as described in the Manual on https://github.com/droidian-images/rootfs-api28gsi-all but with kind help of the members from the Droidian Telegram Group, I was able to install the "Nightly" release to my Volla Phone.
Here are the steps that worked out for me:
* Boot twrp
* Push (with adb push filename.zip /tmp) the three zipfiles
* Format userdata (note that you will lose whatever files you have in your data partition)
* Flash the three zipfiles via the twrp UI in the following order: rootfs, devtools, adaptation
* Before rebooting, pull the /tmp/recovery.log file which may have some useful info... (adb pull /tmp/recovery.log)
Thanks to Eugenio from the Telegram Group for the great support.
I must also underline, that Droidian is definitely not meant to be used as a daily driver at the moment. There are quite a few issues with the phone-functionality, SMS, mobile data etc. BUT it is indeed a very impressive project with a pretty UI
Feel free to give it a try, if you - like me - want to experience the early state of a mobile OS development.