I don't know if it'd be enough of a departure from most jamjars to be what you want, but at thelegion, even though they don't choose to be brought to the game, they're not stuck in a single small place. The team has a main base on a space station the size of a moon, but in their downtime they're free to travel at will through portals to any of the planets in the setting. So alongside the plots taking place on a different world each time, you're allowed to set threads on different worlds with thread partners if you want, and even the SOL events between plots are set in different sci fi settings each time.

They also have a choice whether or not they join the team and the npcs refuse to force them.

It hits your other requirements, too.

- slow to medium but it tends towards medium a lot, especially with plots. Backtag friendly.
- plotty. very very plotty
- the game has a few big casts but most of the taken list is actually LCWs. OCs are allowed
- nonpowered people can get powers and there's training for combat and superhero skills you can handwave in the background
- not murder or memloss

The game setting has it so 14 year olds are considered adults in their society, so we have some under 18s that came in with very little or no combat skills, like Kubo and Dipper Pines, that are treated as just as much a part of the team as the adults. Or at the very least they're not treated as any less capable than the anime figure skaters.