RegularNick

Minimal theme, without a doubt. It increases customization of the editor immensely, I can hide whatever I don't need, focus mode is neat, and generally it feels not too far from vanilla obsidian experience

I am pretty sure that not being open source is not a problem. Again, many people said here, Markdown is pretty much future-proof and mostly independent of any app that you use. The only problems you might encounter are some formatting issues (like Wikilinks, which are not default in Markdown standart) and, if you rely heavily on plugins, JavaScript and TypeScript development, in my opinion, can be a bigger problem, because, you know, these things belong to Oracle