You can already put your PC games on romm and your saves. You just download the game in gog and put it in romm in the windows or Linux platform (I think there is one for mac games also). If you already have romm installed, it might be easier to use it than to have a second service installed.

Maybe take a look at FluidCalendar, I just found it but haven’t tested yet : https://github.com/dotnetfactory/fluid-calendar It’s still in early development but it may have what you need.

Alternatively, you can take a look at Baikal for CalDAV and CardDAV support : https://github.com/sabre-io/Baikal