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.
