I recently started using Obsidian for exactly this reason. Although the software is non-Free, it's just a fancy Markdown editor (but more than that). Since all the data is Markdown - and most importantly, stored on the filesystem _as_ Markdown files - there is no proprietary database. I back everything with Git (and my personal Git server - unrelated to Obsidian - gives my nice Markdown previews and includes a web-based editor). Sometimes I find myself without a GUI and naturally the files all load nicely with vim (and works well with vimwiki). And on mobile I use GitJournal (Android) which is nowhere near as powerful as Obsidian, but for mostly reads and occasional writes, it's seamless Git sync really makes it easy to handle.
And if some day something happens to Obsidian, I'll just go back to Vimwiki. After all, my conversion process moving to Obsidian consisted of the single step of opening my existing Vimwiki directory structure with Obsidian...
GitJournal is pretty great. The automagical git sync Just Works, so everything is right where I need it, when I need it.
I spend 90% of my time on my "daily" notes, so (selfishly) that's where I would focus. My challenges there are:
1. My date format is "YYYY/MM - MMMM/YYYY-MM-DD" in a folder called Daily, which means the files have names like "Daily/11 - November/2021-11-18.md" (and surprisingly, Obsidian accepts that date format and manages the intermediate directories just fine). That crazy date/filename format would need to be configurable and supported for (I think) the Journal feature to work well
2. I don't include a heading with the date at the top of my Daily files (actually, _any_ files), so finding the right note (when I see a bunch of cards that all look pretty similar) can be challenging - at least my directory structure means I can narrow down to the month very fast by looking at the folder names
3. Bonus points for a calendar view to make #1 and #2 easier?
I think if I had to pick the biggest challenge, it's #2 because I end up with a big soup of files that all look the same, where the important thing to discriminate between them is the filename.
I also love the commercial model - I love Open Source, but I don't mind paying for quality, so I get the best of both worlds here. And more importantly, I don't have to spend a huge amount of money just to trial the software for a couple months - I can pay the (small) monthly fee, and if I like it, either pay the "full" fee or just keep paying monthly.
And if some day something happens to Obsidian, I'll just go back to Vimwiki. After all, my conversion process moving to Obsidian consisted of the single step of opening my existing Vimwiki directory structure with Obsidian...