Nextcloud as the server and DAVx⁵ with Fossify Calendar as the client on my phone. On my laptop Kontact, but I haven’t looked at that in ages, so it could be in shambles for all I know. I think I’ve also used Thunderbird with some plugin.
And my Fritzbox router uses CardDAV to populate the phonebook of all connected phones.
As far as I know RetroPie also has x64 images.