adding caldav/carddav next to dovecot

[Marc]

I hope it is ok to post this off-topic question here. I was wondering if there are here environments running that offer next to dovecot also calendar and contacts services.
In the past I was testing a bit with the one from Apple, but I think it is being discontinued because of converting the python 2 code.

I am looking for some experience with a setup provisioning >10k users. Not that I have such requirement, but I want to know if the solution is stable, efficient and optionally can scale. I need something efficient, because I do not have to many resources and high iops available. 
I also do not want any other other 'crap' just the cal (and card) dav solution.

https://github.com/1and1/cosmo
This looks interesting (used 1und1 in Germany?) but not big community

https://sabre.io/dav/install/
This is in php ...