Why is Facebook filled with so much random junk now?
I remember when you could go on Facebook and look through your feed at what your friends are saying, catch up with them, and browse posts that they have made. Now, it's just completely random and chaotic, almost nonsensical. There's no logical sense to my Facebook feed at all. As you can see in the image, they are showing me stuff that I'm not even following. This is not even something that I am actively a part of! It's some random group. So what's the point of following a group or liking a page, if they're just going to show you random stuff anyway?
Basically, the advice is to announce that you're primarily using something else with other groups, so you'll be slow to respond on the other platform. Then be proactive about using the new platform and trying to get individuals to switch to it.
Here are a couple options:
Signal - great as a Messenger replacement
Matrix - great as a general chat/event posting platform; make a room for each subgroup
You'd probably need to be pretty aggressive about pushing the new platform, and confident in being able to resolve any concerns, but that seems to be the best way to get people to switch.