There are pull requests for a sort of super communities, that are basically a view over multiple communities. That would be the only way to merge communities, I think. I don’t see any options on my current 0.17.3 version of Lemmy to move posts from a community to another community, so if you would want to delete a community, all posts will just end up being linked to a deleted community.
I actually really like being this early on the platform, last time that happened was Google+ and that went nowhere. But this time it feels like you can actually contribute to the platform. This can be done by posting interesting things, just chatting in the communities you like, and can even help with the development of Lemmy if you have the skills.
I hope this will be the first Open Source project that I can help, I always wanted to contribute to open source, but that never went further than publishing my own projects (that were way too specific to be useful to anyone else) under an open source licence.