TGStat and the public directories only index groups that want to be found, so the niche and private ones are exactly the ones missing there. The trick is to follow the links instead of searching for the groups directly.
Niche groups get shared where that niche already hangs out. Go to the related subreddits, the X accounts and their replies, Discord servers, and niche blogs or forums, and look for the telegram links people drop. Searching Google for your niche keywords together with telegram invite links pulls a lot of these up too.
Public channels are the other door. A public channel in your niche usually has a linked discussion group or points to sister groups in its pinned message or bio, so join the public one and follow where it leads.
And once you're inside one good group, it snowballs. The members, the pinned posts, and the shared links point you to the next few. They cluster by niche.
Last thing, don't chase directory size. A small active group in your exact niche beats a huge dead one every time, and the active ones are usually the private ones you're hunting for.
Best,
Floqal