Need a calendar server for Mojave

Background: I’m not inclined to spend my time creating and maintaining and updating and securing my own server distro on macOS, which makes me less interested in the approach that Apple is offering. I’ve done that before, and it gets time-consuming to keep everything current and secure and integrated, and to sort through the various and inevitable package update notices as all the pieces and parts get updated (asynchronously). Sometimes on short notice too, as security issues are reported or are being exploited.

Here? I don’t (yet?) know of a good add-on CalDAV server for Mojave macOS 10.14.

Accordingly, I’d acquire an older Mac mini and use that until you figure out where you’re headed with your server, or I’d acquire new or would use use a spare x86 box and install OwnCloud or NextCloud or analogous software installed in that, and that probably booting Linux or BSD. Or I’d use the CalDAV calendar server package that various other distributions either install or offer.

There are various vendors that offer packaged systems with related capabilities. Synology Network Attached Storage (NAS) offers various free and extra-cost plug-ins, including Calendar and mail. Yeah, you already have a 2018 Mac mini, so this NAS approach might not be thr path you want to follow.

As should be clear from the abovd, I’m not planning to continue to use Apple for server functions. Not unless a pre-packaged server distro again becomes available, and quite possibly not then.

I have worked some with Kerio, and it does work well. It’s not cheap, and it does lack some of the services I’ve been using with macOS and Server.app, too. But again, it works well.

It's exceptionally rare for an older release to boot newer hardware. It's usually possible to boot an older macOS release as a guest in a newer release given an appropriate virtual machine, and the Apple software license agreements can also permit that. Check your particular local SLA for details here, though.