If you don't mind separating your mail from your CalDAV there is https://fruux.com

(I use a paper diary, YMMV.)

€4/u/m is pretty steep. I'm currently using fastmail which is $3-5/u/m for mail/cal/card.

It is free if you only use it on 2 devices

Yeah unfortunately the main cost I'm looking to reduce is a family domain which I have ~10 family members on.

Consider etesync, which offers client side encrypted Card/CalDAV.

You can still use Purelymail for mail, and have your mail client provide a cohesive mail/contacts/calendar UI.

We do have CardDAV support, and hope to develop CalDAV soon :)

Good luck with CalDAV - it's pretty hairy! I do recommend joining CalConnect if you have the budget for it - you get access to a lot of experts there. Or at least show up to the calsify mailing list at IETF, we're pretty friendly there too. The edge cases in Calendaring are a right pain.

I would recommend using some existing opensource tool for that and not roll your own implementation