I was going to get a Mac Pro $6000 unit it is worth every penny, when people talk about building their own computer they only look at the processor, or storage or memory. What is the real meat and potatoes is the bus speed and backplane speeds. And apple engineers the heck out of the underling structure. So you might get parts and slap in on a motherboard, the motherboard will never have the speed of the Mac Pro.
I decided to go with a MacBook Pro 16 with 32 gigs of ram, 2TB of storage, which I split for Bootcamp for Windows 10 and the AMD 5500 with 8GB of ram. Got a Razer Core X to use my Vega 64 card that ran in my old Mac Pro before it died. And The whole setup was like $3600 dollars. The nice think about the MacBook Pro it is portable and I can take it to work vs sitting at home not being used. So instead of keep a Mac Pro for 10 years, and upgrading, I decided to get a MacBook Pro and upgrade ever 5 years, so I will get upgraded technology. But still having the option to upgrade the video card in a year or 2 maybe even twice.
This MacBook Pro 16 is awesome and worth every penny. If you are looking for a desktop, that is expandable, tell Apple, I want a Mac mini pro with this kinds of things and feed back to Apple. Apple might grant you your wish next year, That is why you need to use Apple Feedback page to tell them what you want.
Technology has a limited life span so the more time it is not being used, the less value per dollar spent the equipment.
Well maybe when I retire in 10 years I will get a 2030 Mac Pro tower work out of the house from