My point is that none of your four systems go beyond the six core mark ( and if you purchased the 5,1 second hand, likely the tcMP with its higher core count and dual GPUs was also not something you need or wanted ) nor the current iMac Pro ( presumably )
Perhaps you are looking for empty slots in a Mac Pro that you can fill with peripherals of your choice ?
If I bought now, I'd look at an 8-12 core system probably because I'm forced to trade off clock speeds for cores, and software is still very scattershot with actually supporting multiple cores to this day (After Effects lost its buggy multiprocessor rending back in 2014, and it still hasn't made a comeback.) If my daily software was going to be pegging all the cores consistently rather than just intermittently with certain apps and plugins, I'd consider more, but the tradeoff ain't worth it. I'm not a good target for the current SP line because of that.
If Apple had released a revision to the Mac Pro after the 6,1, I'd probably have bought it (or if they'd dropped the BTO prices in 2015 or 2016 instead of the middle of 2017) but when I was looking to buy the economics of getting a 5,1 instead and selling my old 3,1 Mac Pro to pay for most of it were simply too good to ignore.
(It should go without saying I'm not using a G4 for professional work, and my MacBook Pro is mostly about portability—I try to never do any heavy project on it, it's just filling my on-the-go needs.)