Most of these are on Reddit, but there may be a few that I thought could be specific, but not as niche as some of the ones on reddit. (like no community just for one game)

Van conversion/van life

Birding

Camping and hiking

AmITheAsshole

Each state in the US

Longevity

Kindlescribe/e-ink

RestlessLeg (or a chronic condition community)

Woodworking (bigger builds)

Carving (smaller builds, not furniture or big stuff)

Nutrition

NextFuckingLevel

JRPG

Turn-Based RPGs

Action/Adventure Games