I own a Dell T1650 and Dell Optiplex 9020, and yes you are correct about them making everything proprietary, but the motherboard can still be used as long as you have the adapters for it. You will need to DIY, or buy a custom cable from a vendor that makes them for the motherboard. I use a Dell Optiplex 9020 MT motherboard in my NZXT case, I use a EVGA 700BR, a custom 24 pin to 8 pin connector, and a molex to fan adapter cable for my other fans, and everything works great! Also, I added Libreboot support for it as well :D
28 PIN TO 8 PIN ADAPTER: https://www.moddiy.com/products/Dell-OptiPlex-9020-PSU-Main-Power-24-Pin-to-8-Pin-Adapter-Cable-30cm.html
MOLEX TO CASE FAN ADAPTER: https://www.amazon.com/Molex-Case-Sleeved-Adapter-Cable/dp/B08393S29W/ref=mp_s_a_1_4?crid=YC96W76081OT&keywords=molex+to+fan+adapter&qid=1707797145&sprefix=molex+to+fan+%2Caps%2C91&sr=8-4
Full Metal Jacket.