Since the protocol is actually open, you don't have to use the official client.
I'm curious as to why you say it was "never good for one-to-one private conversations". I've been using it for this exact thing for many years, and still find it the best option currently on the market for a variety of reasons (e.g. unlike Signal it doesn't limit the number of devices which are linked to the same account).
I wasn't a telegram user for long so this may have changed but isn't the secret chat limited to device to device? So messages sent from my phone can't be decrypted by the same user account on my laptop?
I just remember thinking, well this is dumb, and going back to Signal (Signal annoys me in other ways, requiring a single phone to be the "master" and other devices to be merely linked. I miss keybase, they had a great system including paper backups)
I'm curious as to why you say it was "never good for one-to-one private conversations". I've been using it for this exact thing for many years, and still find it the best option currently on the market for a variety of reasons (e.g. unlike Signal it doesn't limit the number of devices which are linked to the same account).