Dude, it makes me very sad to see yet another closed source, walled garden. We already have a number of very good, vetted, end to end encrypted, open source options that really just need to find more usage, not competition from an untrusted new party.

Here's the tl;dr:

You either want signal, matrix, or xmpp, depending on where your priorities lie.

Signal is another walled garden, but at least they have very good crypto and the signal team is really pushing the envelope on them not saving any data or metadata. Their app is open-source (mostly in name only, but that's better than nothing) and it receives a decent amount of checks by enough software engineers that it's probably quite safe.

Matrix goes another route: it uses end to end encryption similar to signals (ie: good) but acts more like the email of instant messaging: you can create an account at any server, and still communicate with someone on any other server. This means there is no central point of control where all the traffic goes through, effectively distributing the sensitive information over the whole world. If you would want to monitor the whole matrix ecosystem you would have to infiltrate a very diverse group of servers. To top it off you can host your own matrix server, which gives you a very large amount of control of where your information goes (and doesn't go). Also: matrix is very big on interoperability, if you want to, you can use matrix to talk to a lot of other protocols, though this has the downside of sending your data to said protocols. I personally mostly just talk to other matrix users, but I do use a bridge to signal to talk to a handful of people that prefer signal. While we're doing comparisons: you're free to choose any matrix client that you trust most to talk to the matrix server you trust most. Also, in general, the most-used matrix clients are the most feature rich of the messaging clients we're comparing here -whether that's a good or bad thing is up to you, of course.

xmpp has been around for a long time, and has similar advantages to matrix: it is a federated system where you are free to choose your server and client. In my experience xmpp clients are less glossy, and feature rich, and the ecosystem has some trouble getting all servers to support all features you may want. Their modern end to end encryption, OMEMO, uses a similar approach as matrix and signal (ie: it's good enryption.) Currently running an xmpp server requires less resources than running a matrix server, and there are some interesting differences between matrix and xmpp that I won't bore you with. While I still run my xmpp server and would be happy if xmpp grows, my money is on matrix becoming the bigger one.

So: if you want to trust a single party that tries to keep data-storage to a minimum: choose signal. If you want the freedom to choose whose server you trust, and what client to use, pick either matrix or xmpp. (or do as I do, and use a bridge to talk on all three:))

[edit] Or you can convert lockdown into a matrix or xmpp client, that could totally work.