I installed your rom in twrp. Do you say, that is not (!) necessary?
As described in the install instructions (OP):
You would flash the ROM itself via TWRP, which you correctly did, but I have understood your question being about microG. And microG is not flashed in TWRP, but installed as apps, in fact three apps, which I have seen from your comments you did.
B.t.w. in the while, I found an older version of your rom, I could install and it did't run into bootloop.
Sometimes, the TWRP of this device does not properly clean stuff while flashing. If you experience a "bootloop", please try the following: Enter TWRP, choose "Delete", then "Advanced delete" and check only 'Dalvik / ART Cache' and 'Cache', leave all other options unchecked and swipe. After that, the reboot should work without bootloop.
Puuh, next step, I installed all files of fdroid: microg.org/froid/repo, I could login to my "Konto", but store is not able to install. All files (0.01/0.02/0.10) are not able to install.
The so-called "FakeStore" app of microG is not
an app store, it only mocks the existence of the Google Play Store to apps, as otherwise, those would maybe refuse to work. You should install the app 'AuroraStore' from F-Droid, this is an anonymous frontend to the real Google play store, allowing you to download and install any app available on the playstore (and that without the need of a Google account).
There are not so much apps, I need, but these are from google play, so, I must try to find a solution.
See above: AuroraStore
With all other roms my tab S2 has infinite logouts from wifi and no gps, so, your rom could be the final stable solution
Thanks for the feedback - hope my advice is useful to you.