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.
A lot of speculation that does end with this in the article:
"After discussing her case with experts, Matsapulina now believes her Telegram messages may have been compromised by a form of spyware. When she was told that a hacking device would need to be physically nearby to infiltrate her phone, a memory resurfaced: At times before her arrest, she had noticed an unmarked truck with a dome on its roof parked outside her building. She had even jokingly mentioned it to friends on Telegram. Now, she remembered, as the police were banging on her door that morning, she’d spotted the same mystery vehicle parked outside. By the time the police stormed her home, the vehicle was gone.
Matsapulina has since started using Telegram again."
Most messaging apps are vulnerable on the client side with spyware, no matter what E2EE exists along the way.