As with Threema and SimpleX, Session is one of the most secure and private chat apps that you get today (more so than Signal, and way more so than WhatsApp).
A lot of this is to do with the metadata level (that data which WhatsApp actively resells upstream to Facebook and other providers).
There is no doubt that Session is up there with the best, but this is also why none of your friends will be found there. There is no phone number or e-mail lookup to find users, nor any search to find them. Each friend must choose to share their contact ID with you. You can connect quite easily with a QR code or a contact link, but that is just not how people are yet connecting.
Most users want to log into a central service where their e-mail address or phone number is searchable (at least to those who know it), and that is how people connect and find each other.
Of course that means the central provider knows who you are and can connect that information with your login times, IP addresses, location, etc. Yes Signal tries to minimise that, as does Proton Mail, but others like Google, Facebook, and WhatsApp max out that information to build profiles on you “to serve you better”. Personally, I think WhatsApp and Facebook have taken that too far, by sharing that information, which is why I deleted my accounts with them.
So, yes Session is excellent, but regretfully the world is just not ready to adopt it en-mass (and the same goes for truly encrypted e-mail). The technology is ready, but humans are not ready.
What you can otherwise do, if this does concern you at all, is to rather adopt something like Signal messenger, and for Facebook use a 3rd party anonymising app like SlimSocial. If you are not ready to delete your Facebook and WhatsApp apps (the mobile apps are real data gatherers), then at least create your presence on Signal, so that others friends can delete their apps and can still contact you.
See https://www.howtogeek.com/i-went-hands-on-with-session-an-encrypted-chat-app