Chat with Direct Message in a Webapp

I want to add ability to send direct messages to my webapp. Small social network with 100k users. Requirements:

- Open source or SaaS with SDK (preferably not Google) - Basic functionality is OK (no need for groups, threads) send message, block user etc.

Nice to have - Vue.js component would be great - E2E encryption

I've been looking into couple of options like Twilo, but it's too expensive and I need just a small subset of features. I'm worried about deplatforming. I'd prefer not to roll out my own crypto/protocol implementation like https://github.com/mastodon/mastodon/pull/13820