Some comments:
1. You can play ER on H\H and do quite fine overall, but hard campaign difficulty does make AI more likely to do
insane decisions while VH battle difficulty simply doesn't scale that well with the mod's mechanics. I personally play H/H most of the time for some early game challenge with some factions.
2. Fort battles in ER are overall doable and tolerable. AI will always be AI but the mod uses wooden forts, which have few pathfinding issues nor cause game freezing with artillery as regular forts can. Only real exception are some eastern forts still use vanilla eastern style, which can be somewhat finicky.
3. While nitpicky, St. Petersburg did not exist in 1700 and it was something Peter the Great founded on top of Nyen after the fort was taken in 1703.
4. Provincial line infantry are more akin to garrison battalions and best used as garrison troops in forts to improve public order or to have some presence in case of sudden war.
Curious to see how it turns out.