Your best bet for that is almost certainly beeper.com, which isn't FOSS, but specialises in combining messaging channels and is built on Matrix. Otherwise, you

Your best bet for that is almost certainly beeper.com, which isn't FOSS, but specialises in combining messaging channels and is built on Matrix. Otherwise, you do have to run your own bridges and hope for the best, and moreover the bridges re-encrypt Signal and so miss the point of its end-to-end encryption. On the Element side, we're not focusing on bridging currently.