It was a many months transition, and it's finally done
Fun thing, you can actually make a backup of all* your messages, groups, contacts, etc. So before leaving you can have all of your data in case you need that one contact or something
The final red flag was as that allegedly Russian authorities were messing with people's deleted messages. Not for the first time there are news that they could read, modify, delete, see location, and etc. Screw it, this is unsafe, I'm out.
Also, these days telegram is really at the state of a pile of garbage, bloated, buggy, and shady messenger.
Do what I did. Let everyone you care about on TG that you're closing that crap, with your reasons for doing so. Inform them of your moving to signal, session, whatever. Be clear that, otherwise, they can try calling you and wish them good luck. Close TG on the day you set as deadline. I did that and whomever didn't get a Signal or Session account has to call me. I've never looked back.
My family is all on iMessage. I told them if they didn't install Signal I wouldn't reply to their texts.
At first, whenever they texted I would just reply with something that looked automated like "This user is no longer available via text message. Please install Signal if you wish to communicate."
I did something similar and just sent a link to Signal when IPhone friends and family SMS'd me, worked....eventually :) (am on Android)
That's freaking epic. Love it.
Like that, also, a few months prior to the deletion turn off the notifications, and come there every few days, so people need to wait for your reply for days, and when you come you say "oh, I'm not using tg, I switched to signal/session/simplex/bird mail"
These are allngreat suggestions. Another huge advantage is that this help detoxify from the constant pinging with others.