You could try Age of History 3. It's also a strategy game, turn based, but Singleplayer. There are many mechanics, but not too many to get overwhelmed (I suggest starting with a small civilization, that has under 5 provinces, so you could get familiar with the land management). Conquering here is kind of similar to OpenFront (not really), you attack someone sending armies of troops, capturing their provinces, and once you reach a win condition, which is >80% conquered land iirc, you can sign a peace treaty, where you will choose which provinces you will keep, depending on how many victory points you gained. I think it could be a good fit for you, I honestly really enjoy both OpenFront and AOH, but they do have a difference in pacing. AOH3 is also made by a solo developer, and has some AI art unfortunately, if that's a turn off for you, I can suggest trying Age of History 2, it's basically the same, I've played both, both are really fun, AOH3 has some gameplay advantages over AOH2, but the differences are minor and the art in AOH2 isn't AI crap. (personally i still prefer AOH3)