When I try to replace my texting app with a FOSS version, I lose some of what I would say are critical features, attaching large files, like/reacts to messages, control over notifications. The dealbreaker to me here is the ability to attch large files. But maybe there's also a way to send encrypted messages using SMS/MMS/RCS?

I'd like a texting app the works over SMS/MMS/RCS and uses proton drive to attach files similar to how Google Messages uses Google Drive to attach files.

The links would need to be public links since its not sent over secure servers, but you could add similar security enhancements as you do with password sharing: links expire, password protection, and set a number of downloads limit.

The messages themselves could follow the same protocol.

It would be nice if I could send an encryption key using Proton Pass that would install on the receiver's device and allow for automatic decrypting of password protected encrypted texts when they click the links from non-proton apps if the key is installed on the device.

Lower priority: fully customizable notifications, sending/receiving reactions with Google Messages and iPhone users, and E2EE messaging with other Proton Messages users.