@kundifon You can't install this Lineage OS on Huawei P9. You know, European P9 original Firmware is based on Android 7.
Rebranded to Chinese model, it's based on Android 8.
The LOS you have mentioned, is based on Android 11 (LOS 18) and wouldn't work. It would send your P9 to nirvana with no return, if you could install it at all.
Only alternative to the MicroG Rom you have tested, is the Rom made by OpenKirin (including GAPS).
There Website is offline.
But the Firmwares for P9, which are mentioned here:
https://xdaforums.com/t/roms-treble-openkirin-aosp-collection.3828806/
are still download able from OpenKirin Telegram Channel:
https://t.me/OpenKirin
You can only install the ones for EMUI 8! And those ones, which are listed in the XDA Thread.
The only exception is the Omni Pie Rom, which is based on Android 9 but backwards compatible with Android 8.
The installation process is described here (in German, if your native language is English, please use Google Translate) :
https://www.android-hilfe.de/forum/...u-al10-rebrandeten-p9-und-p9-plus.901530.html
The important prerequisite is:
The phone must be on Android 8 with working network patch.
You need after that the EMUI Stock Recovery (not TWRP).
You can switch to Stock Recovery by flashing this script with TWRP for Android 8:
https://www.androidfilehost.com/?fid=1395089523397921866
After this step you must download and unzip the file (with 7zip) and get a system.img for flashing only in Fastboot Mode.
Once Rom flashing is done, you must boot to Stock Recovery and doing a factory reset from there. That's all in a nutshell
*****
Not necessary for AOSP Rom, but just for your question about TWRP for Android 8:
you can't do a factory reset, because it hasn't access to /data cause of the missing Decryption. And there's no any other TWRP.
@zgfg found a way and described it here in this Thread to be able to get access to data by format data (several steps are required to manage this without bricking the phone.
Perhaps he has stored the link to his post.
At the moment it's really difficult to search this thread and find it.