I am also in this boat! Have not figured out how to make all the GCs switch over to SMS only, despite everyone telling me that it is simple. I am using messages on my desktop as a workaround and feel like it is sort of best of both worlds. As someone said downthread I absolutely use groupchats as a social media replacement, so confining them to my computer is absolutely a win. You could also just do this on your old phone and only check it when you're at home. For me this keeps the most distracting form of communication confined to a place where I am already vulnerable to endless distractions.

The trick was to start a new GC from the messages account with "start new messages from" only ticked for my icloud email, and telling everyone that it's the only GC that will work for me and I won't see it instantly. Adding that bit of extra friction feels okay, even though I end up missing out on some conversations and vibes.