You can basically get rack mount level performance from:
Supermicro SuperWorkstation Tower Servers
Lenovo Thinkstation P Series Server Towers
HPE ML series Tower Servers
Dell Precision Tower Workstations
In your situation, I’d be looking at ebay, serversupply, or other used hardware resalers that offer 2 generations back hardware. Used DDR4 based systems are abundant and cheap enough, go that route.