I just did this.
The tablet had Android 11/R on it (as required by the /e/OS install instructions), but the attempt to get access to the OEM Unlock by updating (as the instructions say) ended up putting Androidn12/S onto it. 
I tried to find a way to get 11/R back onto it (with Heimdall, stock ROM and even a Linux version of Odin), but just gave up and did the 9 steps listed in this post. Essentially, I “stepped down” from 12/S to 11/R using the Lineage recoveries and ROMs, then flashed /e/OS onto it. It worked!
The OEM Unlock was a big problem, so I ended up just doing it from the Download Mode ([VolUp]+[VolDown] & plug USB cable into device while cable is connected to computer) and selected the Unlock Mode to unlock the bootloader. It proceeded without the headache of no OEM Unlock option under Developer Options after that.
A trick I learned from @chrisrg is to flash the next-to-most-recent ROM version (in this case 1.2 as of this date) and then do an OTA update to the latest. This takes care of any A/B partition issues that might exist on the phone/tablet.
It’s been working nicely for several days.
Good luck.