You can have small collaborative teams building cool impactful things inside a big company. It does take a particular set of conditions to pull off, a mix of vision and ambition by the engineers as well as agenda and trust by the management I'd say.

Niches can have some deeply interesting stuff too that can keep you busy and interested for a long time, but that's more up to personal tastes whether any given niche would be a match for someone or not.

I won't go into specifics, but I've seen first-hand the achievements of such a team in a particular niche I happen to find interesting. Unbelievably cool and unique tech just hidden in plain sight and a joy to work with under the hood. On the other hand, it can feel like being on the other side of a treasure hunt, where you secretly hope one day someone outside gets nerd-sniped inside your particular niche, follows the breadcrumbs out in the open and pieces it all together, just to see what madness they could pull off.