That's not going to work out well. mITX is a niche product and many of the best mITX cases are not on pcpartpicker.com, because they aren't sold in newegg or Amazon, you'll only find some of the more widely advertised cases there. But it can be a start.
Best bet is Google. Find places like Optimum Tech, pcmag, ign, pc case gear, techinfluencer etc who do 'best mITX cases' etc. You'll need to start with researching case designs, who uses split cases with seperate gpus and riser cables and who uses more standard cases, which are fully modular, which aren't etc and then with that 'idea' of what you want the end result to be, start aggressively researching that design.
My case is an nCase M1 v6.1, originally a very small outfit, but since picked up and built by Lian-Li. CoolerMaster now has an almost identical clone, that's cheaper, but I prefer the Lian-Li quality and construction. Jayz2cents has done a video on the CM design, Optimum Tech has done several on the nCase. This is where the majority of ideas will come from, seeing it in practical usage, actually seeing what is recommended and almost universally used, and why.