Can I update CPU, memory and SSD of my iMac Pro?

Here is a detailed listing of the iMac Pro systems

I wouldn’t be to quick going for the 18 core. When this system was designed the Intel chip logic was still struggling with its architecture.

Unless you are rendering or working in the medical field working with CRISPR having more cores and loosing clock cycles doesn’t offer most people the performance they are thinking would be gained.

128GB of RAM likewise is also no slouch! Review your Apps and work, even with my 2013 Trash Can doing heavy graphics work I haven’t needed more RAM. But onboard storage is a never ending desire for sure! But Apple went proprietary Flash modules, not a true SSD! Review this iMac Proの分解

So the bottom line here is other than the RAM, the Cost - Benefit is just not there besides the risks of damaging something in the process which would not be good.

So while it’s nice to have an expensive Bugatti racing car, getting serviced or even driving it on the streets and dirt roads won’t push it to what it was designed to do.