Earlier quoted context omitted.
Signal has multi device sync now, and it's cryptographically secure. It works by sending Signal messages to the other linked devices. This enables Signal-Desktop (the Chrome app that is now in beta). Sync is only implemented in the Android client at the moment though.
That's quite literally multi-device >>sync Seems like a low effort solution and would not satisfy my requirements, but it probably works for some people.
[0]: https://gist.github.com/TheBlueMatt/d2fcfb78d29faca117f5