I use pcpartpicker. It has a pretty good hardware compatibility matrix and pulls prices from a variety of stores online. Doesn't do a perfect job with packing hardware in small cases, but it warns you that it can't guarantee a fit. Your skills from 20 years ago will serve you well: download the manuals for the motherboard; they're more accurate than the specs listed on any other website.

I've got an asus rog zenith ii extreme. I had an MSI board before that had trouble with the RAM and had a back and forth with MSI on before just sending it bacj.

As the other commenter pointed out, pcpartpicker is the way to go. My method was to find the CPU, chassis and amount of ram I wanted, enter them into pcpartpicker and pick out the test of the components with their help for compatibility except ram. When it came to ram, the motherboard providers have a list of "officially supported" sticks, down to the model number. I went through that list checking reviews and availability and ended up with some g skill ram.