I sync Joplin to S3 and I used to also use the iOS app. I lost my phone while on vacation last year, but I was able to lock it down and it was encrypted. I setup a trigger on AWS to let me know if that sync ever fired again and it never has, so no data was leaked. However, it spooked me enough to never want to keep a full sync of all my notes on the phone again. It's just too many personal information vectors of attack.
So, I've been living without Joplin on my phone since then, but I would like to be able to use Joplin on my phone with only the notes I choose to be sync'd to it. Is it possible to do this?
Look into profiles. You could have one for regular notes and one for sensitive notes. You have both on your Desktop app and keep switching between them as needed, and on your Mobile app you only have your regular notes profile. Each profile has its own sync target.
I have checked into this and it is an option, but the issue I have with this approach is that you can’t easily move notes between profiles. Switching profiles is a PITA, and you’d probably be better off running two different instances of Joplin, but that’s also cumbersome.
This is what I’d prefer to do. I imagine it would be a simple addition to Joplin.
What would be ideal is to be able to tag a note or notebook in the (main) instance and have that replicate to another (mobile) instance without the need to intervene. Editing a tagged note on the (mobile) instance would write any changes to the same note on the (main) instance. So you’re only seeing and editing notes on your phone that have been tagged with a specific (mobile) instance name.