Judging by your name and the high price of electricity you stated, I assume you live in Germany?

If so, mabe this could be an interesting option to consider in search for your server: https://www.ram-koenig.de/Gigabyte-MJ11-EC1-AMD-EPYC-3151-4x27-Ghz-Mini-ITX-Mainboard-ATX-Adapter-Server

It definitely is "a project" as it is a custom board from gigabyte, designed to be used in a Mining rig. It abides by ITX standards however an ATX adapter is required for power delivery to the board.

I got mine from Ramkönig, bought 32GB of registert ECC ram on eBay (27€) so the total for the server (if you have a case and HDD) for me was under 100€.

Power consumption is at around 20-30 watts in idle and according to the tdp of the CPU 45W higher when under load.

What is also interesting is the option to use an M2 SSD on this board. It also has 4 SATA ports.