What are my choices for these app replacements? They absolutely must work on both Mac and Linux and it would be even better if they're core Purism apps for both.

I did see some Purism 'replacements' for iMessages but don't remember about Facetime. Those are easy to transfer to any given Linux distro, but what about replacements for FaceTime (video chat) and iMessages (text chat)? In general, most of my high-use Mac apps aren't really Mac, anyway - LibreOffice, Firefox, Signal Messenger, and a few others. I have about a year and a half (this time around) with Linux (Ubuntu 18.04 LTS) and I'm fine with using Linux - and might be ready to try a new Distro. I'm considering moving out of the Apple ecosystem to a Linux laptop. One of our two Mac laptops is getting a little old and will in the near future stop receiving security updates.