I do not like it (I prefer Matrix, where accounts are not tied to phone lines), but I am ok with Signal, as it is open source, it is always e2ee and using it really offers privacy.

Telegram, on the other hand, is not open source, nor does it do e2ee by default. Having to explicitly select "new secret chat" ultimately means the non-technical inclined can and will use it wrong, getting no privacy whatsoever.

Therefore, I cannot support the idea that Telegram is any better than WhatsApp or Discord.

Regarding "open protocol", I suspect that what you think it means is not what an open protocol actually is.