The server use case for Mac Mini is a huge compromise. Sonnet Tech [1] makes a 1U rack enclosure with a PCIe riser connected internally to the Mini's Thunderbolt port.

Internally we had one used for our company's OSX Server running profile manager before going to Jamf Pro.

[1] https://www.sonnettech.com/product/xmacminiserver.html

Ok. But then, which Mac would you use as a build server? Mac servers no longer seem to exist, iMacs and Mac Pros are not stackable, you can't build iOS/OSX apps on a standard linux box, and laptops have the same issue as the Mini. That's why a Pro-focused Mac Mini, with even a mid-end desktop CPU and reasonable RAM would be ideal here, and a Mini with a modern laptop CPU would still be a vast improvement of the current situation.

I'd say this use case is ideally suited to the Hackintosh but I haven't attempted it. I mean there's all that server grade hardware out there just waiting to be hacked to work with OS X.