never played 4 or 5, but from reviews at the time it sounded like 4 and 5 were just lower effort games coasting on the reputation of 2 and 3
Might and Magic is a fun setting because it's secretly SciFi and you would never know it from the Heroes games as far as I've ever seen.
But a bunch of the Might and Magic RPG games start with sword and sorcery and end with laser guns.
3do did rush heroes 4 though, and one of the biggest complaints about it is how bland and lifeless it feels compared to 3.
If you haven't already, you should try the Horn of the Abyss mod for Heroes 3. It's great! Adds tons of qol stuff and has a new campaign and faction.
That got mostly cut during development and they never went further down that route. Horn of the Abyss is attempting to bring that faction back to life with Forge.