> It sort of stopped altogether when we started getting responses like "hey, don't call me without scheduling a call with me before" or getting a text 3 days later "hey what's up, I don't want to hang out".

This is a good thing!

It doesn't (necessarily) mean that person doesn't want to be friends or doesn't value your group; it means they feel comfortable telling you how they feel even though doing so is a mild violation of social norms.

If I were in your shoes, I'd just make sure they're not accidentally booted from the group chat (etc.), but otherwise just leave them be. Maybe a couple of times a year mention something like "We're all going to ___ next week, if you'd like to join. No stress!" just as a keepalive, but otherwise let them do their own thing.

I have several very close, long-term friends that I've not spoken to in months or years, because that's just who we are and where we are in life. If any of them called me in an emergency I'd drop everything to help them, and I'm 100% confident they would do the same. We _have_ done that for each other before.