Back with newsgroups the general rule was to go from general to specific. You start with a general discussion group and when discussions about video games get annoying you create a games group. If then there are too many Baldur’s Gate discussions you create BG. If they are dominated by Baldur’s Gate 3 you create a Baldur’s Gate 3 group. If everyone is fawning over Withers you create a Withers group which of course will be flooded with discussion about the Withers’ tits mod, which shall get its own group.
Meaning you should create a group when demand is there and not the other way around.
The issue is that community don’t get big enough because people want to replicate the niche communities from the get-go, without ensuring a sufficient user base for the niche