GMKTec G9. Four NVMe slots, plus some internal memory to deploy TrueNAS. It only has 12GB of RAM, but has two ethernet cards, acceptable cooling and performance and has a small footprint.

SSDs shall be single sided and gonna need heatsinks, but I believe it works well and is not tied to any manufacturer for anything.

If you want go all-out get ASUSTOR's Ryzen based systems. You can use the stock firmware or disable it without wiping it and deploy TrueNAS on it. It's a beast.

TrueNAS has containers and applications and VM support so you can run any service you want on it.