I hesitated to comment on this, thinking that there are better informed members than I that would. Methinks this is a misuse of the concept of Groups and Sub-Groups and what you are really in need of is a structure for messages. You seem to be trying to structure content through Sub-Groups, a facility that is intended as a means of splitting up members of the mother Group into focus groups. But your content is seemingly of interest to all members of the main group so there is no reason to break the membership into sub-groups.
Could you not achieve the structure you want through #hashtags, e.g.,
#meetings #2018 #april for all Topics related to the meetings in April, 2018
#meetings #2018 #may for all Topics related to the meetings in May, 2018
The number of hashtags is quite manageable:
months = 12
meetings = 1
years = 1 per
Well, maybe there is a reason for sub-groups - getting posters to practice the discipline of using the right hashtags. But then you will have the same problem getting them to post in the right subgroup. Fixing hashtags is probably a lot easier for the admin than moving posts between subgroups.
Searching for connections among different months will be much easier if all the messages are in the same group. You cannot search across all subgroups.
Now there might be some enhancement along the lines of Categories that would help structure your content but for both subgroups of subgroups and Categories you will have to wait. What you have to work with, in addition to hashtags, are Databases, Wiki pages and structured naming patterns for Topics. I think you should explore further what you can do with the current features - no waiting.