Well, it wasn’t that simple, but in the end I have /e/OS running on the Gal Tab S6 Lite 
The stock ROM install just wouldn’t work.
The steps:
- as above, install Lineage Recovery for 12/S using Heimdall
- install Lineage 19.1 (S) from Recovery
Reboot - install Lineage Recovery for 18.1 (R), using H
- install Lineage 18.1 (R) from Recovery
Reboot - install e-recovery-1.2-r, using H
- install e-1.2-r from Recovery
Reboot - OTA update the OS to 1.3-r
- Chugging along
Probably ridiculously over-engineered, but this seemed the safest way to “step down” from A12/S to A11/R (and it uses @chrisrg’s trick of updating to the next-to-most-recent version then doing an OTA to the newest, so hat-tip 
).
Now configuring…