Hot take here; so PLEASE do not replyunless you’re the OP.
Maybe you don’t have to switch. At least not immediately. Ultimately someone will make a stink out of switching. Again!
MProto isn’t the best cryptography around; but it does provide some implementation of some low level privacy. I wouldn’t trust a credit card number to it; but it’s not worthless. It is however heavily disliked by people who understand cryptography and value privacy. But Your friends don’t care! That fact is irrelevant to them emotionally.
And this is where the problem starts…Your friends have gotten accustomed and attached too much to Telegram and it’s many eye-candy features and smooth polish.
This is where you need to “Sour The Milk” and wean them off Telegram.
Perform your usual setup of a new group. Use Signal or Matrix (usually this will be though Element). Then Force anyone who decides to be stubborn on Telegram to use the “Secure Chat” feature on Telegram. At least then the stubborn holdouts won’t be causing you excessive privacy issues.
Then lastly just transition to your new platform and pay exclusive attention to the new one and let people trickle in.