This is mostly just for discussion, but this is my PC's current state. I do want to do a full custom watercooled setup sometime but I'm wondering if anything is screaming out "upgrade me, I'm old". I mostly game and do CAD design/3D printing. Some photoshop and After Effects work every now and then. What would you upgrade?
I'm not sure what you mean by this. CAD and 3D printing are two separate processes. You make a design in CAD and then bring that file into 3D printing software (known as a slicer), which converts the model file into a gcode file (basically a list of instructions that the printer interprets for printing) that is given to the printer. The 3D printing part is mostly handled by the printer itself. Slicing the model file is the only part done on the computer. You can also just download files to print and never even use a computer if you don't want to design yourself.
I sometimes download premade files to print, but more often that not I make designs myself in CAD, which I then print. But yes, you are correct in that the 3D printing part isn't memory intensive. But I do a lot of CAD design, which is.
The 3D printing part isn't as it doesn't really involve a computer if you don't want to. But if you do CAD design, then yes obviously. The 3D printing slicer software is still 3D software, but you can't do as much as a full blown CAD program.
I use Fusion 360 for CAD design, which uses multiple cores.