I flashed without any issues.Flashing only the ROM after a full wipe works, but flashing Gapps linked in first post with it results in getting stuck at boot screen.
So, which version of Gapps works?
EDIT:
The versions of Gapps I've tried are:
open_gapps-arm64-10.0-nano-20191006-BETA.zip
open_gapps-arm64-10.0-nano-20190920-UNOFFICIAL.zip (from the mirror)
what i did:
wipe, system, dalvik, cache (not data and internal storage) in TWRP 3.3.1-Codeworkx
FW 9.0.9 - wipe cache
flash 17 build, open_gapps-arm64-10.0-nano-20191006-BETA.zip, Magisk 19.5
wipe cache/dalvik
reboot. booted normally. all apps intact!
