It’s not a matter of finding a more private app. It’s about keeping a group and have an opportunity to expand it, reach more people who would need to be a part of it. Any app in Russia that is not telegram would be too obscure for that. For now it’s a perfect balance between privacy and reach.
It does and it sucks. But you can still have adequate protection because knowing your number won’t help authorities much. They have to find you in some group they deem illegal for anything to happen. They must find your account first, then add your number and see if that’s your account. Telegram did some improvements on that issue at some point, so it should take much longer and more resources to do.
Also it’s relatively easy to get a sim card not tied to your passport in Russia. Also using a cheap sim from another country is also an option, since you can set up a cloud password so that even if someone has your sim they won’t access your data fully.
A service requirement of a telephone number is not a honey spot. But sure some groups are honey spots. Yes, authorities mainly operate within the service. It can get to overwhelming extent but that mist mean they don’t have real backdoor-like access.