You can host the data and/or the app that can be accessed cross-platform. If you're on someone else's PC be it linux, windows or mac you can pop open a browser and visit a url to access your app with the stored data. Or you could host the data and access it using the clients on multiple/different systems.

what low-maintenance are you referring to? I have a server at home running proxmox and a container for docker. It hosts a website (caddy), website comments system (cusdis), sync, calendar, mail, mqtt, gps tracking, note taking, at one point i had home assistant but i dont have enough IoT to make use of it so i just stopped it. I check on stuff like once a month or maybe two months.

I can't remember if it was Proton, but i remember reading about some laws being approved where (I think) Proton servers are that would make them comply for disclosure if necessary. Not that it matters much unless you're doing illegal stuff, i believe i found this while researching the differences between Tuta and Proton.