Sure, I can make a mod copy of sunset_invasion.txt, keeping the DLC dependency in the triggers but scaling back the event troop spawns... just give me a good idea of reasonable (or reasonably challenging) troop size.
Assuming they come at at the 13th century, the largest realms had around 50-60k. To account for the AI's assault on by then high fort-level castles (i increased fort levels a lot while trying to keep garrison sizes approximately similar with levies), an initial of maybe at most around 90k. It should make the fragmented western states capable of pushing them back after they lose steam in brittania/hispania. That's a rough guess though.
Also, I checked the save and it seems that most of your early republics have been replaced by feudal dukes with the same liege by that time (if their realm wasn't conquered). Should I try buffing the early city upgrades to help them survive revocation wars (assuming those happen)?