Given that these topics are relatively niche, have you tried reaching out to the moderators of the duplicate communities to see if they would be open to consolidation?
I assumed they were aware of each other and simply differed in moderation style or the type of user they wanted to attract.
I have, however, been toying with establishing a presence in the fediverse for conlangers/conworlders to congregate; likely not Lemmy, but NodeBB, as I like the more permanent nature of forums compared to more modern social media. As for non-federated conlanging communities, there's the CBB where I hang out most, the unrelated ZBB, both running phpBB. But the granddaddy of conlangery on the internet is probably the conlang mailing list.
I can only vouch for the CBB, as I've merely lurked briefly on the ZBB and have never checked out the mailing list, though it's on my todo list.
Promote your favourite communities and ask about communities that you are looking for!
The communities can be from anywhere, including Lemmy, Piefed, Mbin, NodeBB, Matrix, Signal, etc.
๐ฃ๏ธ Sharing a community:
When linking to a community, try to use the universal format: !canada@lemmy.ca
๐ Searching for a community:
You can also check out this guide: How to Find Communities (fedecan.ca)
Rule(s):