This app was mentioned in 6 comments, with an average of 1.50 upvotes
> Do you still use syncthing to ensure the files are on multiple devices?
Actually I don't even go that far, I just use dropbox.
> How do you search the content of the files, especially on mobile devices?
I realised that I don't search as much as I thought I would, but when I need to I either use grep or the search function of notepad++ (since it's what I use to type my notes anyway) on my laptop or aGrep on my phone (it's ugly but it does the job).
Use aGrep, (or grep natively if you know how), to search within text files. Hasn't been updated in forever but works perfectly.
That said it's not open source, but it is free and adfree.
https://play.google.com/store/apps/details?id=jp.sblo.pandora.aGrep
agrep last updated March 2014, but still one of the first apps I install onto a new device.
For searching the contents of large numbers of text files it's the best thing I've found.
Would using grep be of use to you?
How To Use grep Command In Linux / UNIX With Practical Examples
I'm using Obsidian for Android so I'm experimenting with ...
aGrep Android app | png1 | png2 | png3
I can then easily open them in Markor which looks at the same vault folder as Obsidian.
[PS: for compatibility I've disabled [[Wikilinks]] in Files & Links, yet I still get the advantages of [[]] ]