Have a look at https://github.com/rochus-keller/crossline/. I use it for the mentioned purpose (besides Cycling/GPX) since more than a decade. Originally I used Netmanage Ecco, which was discontinued and didn't support all features I needed (i.e. cross-references, more than one outline, etc.). It has an integrated database using the SQLite backend on top of which I implemented a kind of object database (extended entity-value). It's written in C++ and "hackable".