Vendor Name: Teracube
Device name: 2e
Device CodeName: emerald
Version of /e/OS or Stock which existed previously:
1.12.5-r-20230705306756-dev-emerald
Is the device Rooted / Not rooted: rooted
OTA update failed to install. After a few attempts I exported the update to SD card and flashed from recovery. That went okay.
Alas, that meant I missed out on the Magisk OTA update also. No problem as I had the 1.13 fastboot files ready just in case updating totally failed.
With 1.12.5 I had downgraded microG. Figured I’d have to do the same with 1.13 but adb remount repeatedly failed with /system, /product, and /vendor. The mount command also failed to mount /system. Always reported as not mounted anywhere so can’t be remounted.
So after deleting some microG dalvik files under /data/ I booted into recovery and was able to do what I needed in /system from there (/mnt/system/system).
When I fastboot flashed 1.12.5 per the instructions (things had changed since the last time) it included logo-verified.bin. A problematic item responsible for dm-verity corruption boot message along with a poweroff countdown.
Ultimately I wound up dirty fastboot flashing 1.13 along with Magisk patched boot image, logo.bin, and using the --disable-verity --disable-verification flags when flashing vbmeta.
Now everything is as good as before 1.12.5.
Fun stuff. 
Interestingly I was sure I replaced microG before fastboot flashing thus getting 0.2.28 again but 0.2.27 was present after boot. Mozilla and Nominatum backends were gone but the others were still present. Installed them.
After awhile a check with SatStat showed great location results. Tiny red GPS circle on my house and a small blue network circle covering my house and the building next door.
Everything else in the build seems good to go. App Lounge acts odd but then when hasn’t it. 
EDIT: Regarding failure to mount /system I never thought to try mounting the root directory instead. From what I read somewhere that should work(?).
mount -o rw,remount /