If you loose heroes new ones are available for recruitment at similar levels (maybe 1 or 2 levels under your current average for the party).

I might make levels fewer and more impactful to avoid too much fiddling with points (since you have 6 heroes) and make level up more significant. Level 10 might be max for example (darkest dungeon heroes has obnly 6 levels for example).

In some games (such as tabletop d&d 5e) your character gains almost 100 % afficiency, at least during the first level up, but that of course demands the game to be balanced around this. But that is way too much for a game like this, im just saying levelling impact is very different in such a game and wow or diablo 3.