Maybe post something to those "dead" communities. Often it's just missing content, but people will engage with the occasional post, since the subscribers of that community (mostly) do still exist.

I think it's a mistake to look for niche communities on Lemmy. There simply aren't enough people to fill those communities and they wither and die. Many people have made that mistake, which is why it's harder to grow Lemmy as a whole. My advice would be to just ask in a larger active community which is somewhat in the right direction. Then you have the best shot of reaching people that might have answers for you.