I also use Simplex, but you see, a messaging app has the purpose of... messaging people.
Basically, no one uses Simplex (a part of a few selected chaps here on the forum).
There are a few outstanding apps out there (I like Keet.io very much), but if nobody uses them, there is no point.
I know what you are saying but once upon a time nobody used telegram, and before that nobody used viber, and before that nobody used skype, etc.
If you want to continue using telegram than you have to agree with their terms and accept all the risks,
just keep in mind there is zero privacy with telegram app, and everything you wrote including used phone number can be leaked one day.
But even at that there are way you can get to adjust or set up your telegram so that situation like this don't occur and moreover there is always the button of
block user maybe not hesitating to use will save you the time and stress to worry about some hypocrite bugging and invading your privacy.
But still telegram is arguably the app with most scammers and scam related events too and that's why one need to careful in conducting business there although almost everyone now uses it so even your clients might want to reach you through this means, so don't let one stupid dude change you from using the app if it's beneficial to you one way or the other.