IMO, Briar is the best option. I recently started a project to connect users in the Briar private groups and forums. We just need to get enough users active to get the network effect working for us and not against us.

Now I'm wondering how much of the price we pay is due to the cost of manufacturing and how much is just a bunch of middle-men driving up prices. Maybe this is just a case of a bunch of shell companies organized for military purposes but I'm going to guess that the practice is relatively normal enough that it didn't raise any red-flags among the other firms involved.

So what is the best way to store things for the very long term?

There is a desktop app. I am hopeful with EU cracking down on Apple will eventually result in Briar becoming available on that platform. I am working on an idea to connect people on Briar for use of the private groups and forums so you might check back with me in a few weeks to see where that's at.

Now that most of my friends and family are using it, I'm on Briar Messaging every day. Since there are no central servers, is entirely encrypted, and runs on the Tor network, I think it is probably the most secure messaging platform out there. It also has private groups and forums but I am not yet involved in any of those outside of a couple of small ones that are just for sharing family news.