Creating a DC input ATX power supply using buck converter
You're on the right track. Take the next step and consider using a 48V source, following emerging practice in data center technology.

More here: https://blog.se.com/datacenter/2018/05/24/12v-vs-48v-the-rack-power-architecture-efficiency-calculator-illustrates-energy-savings-of-ocp-style-psus/

48V ATX power units are available from various sources in the same form factors as for standard IT gear.

Going even smaller, this one in particular is a module that plugs directly onto the motherboard ATX header, accepts 6-32V: https://www.mini-box.com/DC-DC