Re: adding caldav/carddav next to dovecot

I don't have exp with 10k users, but I switched from radicale (which worked great!) to Nextcloud which has cloud functionality in addition to Card/CalDav interfaces too. Perhaps Nextcloud might fit your requirements. https://nextcloud.com/globalscale/

NextCloud's a fine solution. but, the OP's requirements specifically included:

"I also do not want any other other 'crap' just the cal (and card) dav solution."

i suspect it's a more resource-heavy solution than Radicale; but that's an unmeasured WAG. ymmv.