If you simply bought a new DDR4 Z790 motherboard (
this one as an example) and tube of thermal paste, you could swap that out, keep your ram, reassemble/reinstall and you're done. Big pain but not really another option. Then we can help you undervolt the 13900 so you won't throttle out, turn off the thing where cores park themselves, and your performance would be much improved.
If you want to go that route, you'll want to do a few things first like create Windows install media, etc. We'll provide links for all that. The important thing to know is: did this place give you the Windows License Key, AND, which PC case did they use? The license key is still important because when Microsoft detects a hardware change, often they won't activate Windows 11 from your Digital License in your account. This has happened TWICE to me after MSI Mobo BIOS updates. I simply enter the key and it's good. Still a good argument to by the DVD, lol.
I know this is a lot to throw your way. Think about and and see what others on here suggest, and one way or onother we'll get you... um... up to speed.