I use CoMaps for this. It took a little bit of time for me to get used to it but I now find it so much better than Google Maps bookmarks and I have moved everything over. CoMaps allows a wider range of pin colors and you can add detailed descriptions with each bookmark. No automatic cloud backup but I just export a backup to Proton Drive every couple of weeks or so, takes less than a minute to do so.
A useful app for this task is Geo Share. Helps opening links from Google Maps in other open source map apps.
Good luck finding your solutions!
I can’t help on the open source side but I find Komoot to be more user friendly and open. For example you can download GPX files without a subscription to then make trail bookmarks in open source apps like CoMaps.