https://build.twrp.me/twrp-3.4.0-0-montana.img - should fix most issues, forceencrypt disabler (if flashed) should always be flashed last after ROM and add-on flashing. GApps seems to persist, not definitely tested yet. Feedback appreciated.
Clean flash instructions:
Boot TWRP
Format system, data, vendor, cache, dalvik
Reboot to recovery
Flash LOS zip
Reboot to recovery
Flash any add-ons (if you want to)
Flash forceencrypt disabler (if you want to)
In order to get GApps to persist across dirty flashes (warning: not tested, probably only needs to be done once if you already had LOS installed before, if clean flashing this shouldn't be needed)
Boot to TWRP on existing SAR build with GApps installed
Dirty flash LOS SAR
Flash OpenGApps
Reboot to device
See if OpenGApps is there
Reboot to TWRP
Dirty flash LOS SAR
Reboot to device
--> GApps should still be there.
Update: GApps indeed does persist across dirty flashes. Please use the linked build.
Also, for anyone curious, a reboot to TWRP is required after flashing LOS in a clean flash to flash other add-ons so that TWRP can apply needed changes after recognizing the system as Android 9 SAR (since before flashing system partition is empty and therefore detection of installed system is impossible).