@mvilain That depends greatly on what ypu’re doing with that old Mac Pro.
Given the budget, probably an incremental migration of services from that Mac Pro or #Xserve to Synology, where Synology supports the needed add-on services.
I’ve done similar migrations. For those, Synology and #Ubiquiti together can provide most (all?) needed network services.
Past that, I’ve used a mix of other network and server giblets. Ghe occasional OpenVMS server too, in the before times.
For an app needing a small back-end, I’ve implemented my own, built back-end platforms, and would now look at using Parse Platform or similar.
Haven’t used Synology containers or VM guests as yet, but what I’ve already used as worked as expected.