> I guess I would like to have a solution that not only synchronises across all platforms (Linux, macOS, Windows, iOS), but most importantly is secure as my notes can contain sensitive information. If there is an open-source solution for this problem, I have no problem with self-hosting such system.

If access across machines and operating systems is a priority for you, you may want to set up a private wiki and ticket tracker, either on your own machine or with a cloud provider.