Jeepers, guys... I leave for a day and come back to this...!

Just for the record, and as someone noted, I did update my budget... I'm not looking to spend a bunch of money, I don't typically throw money at tech (just look at my systems...) I made that mistake with my first PC (the one being replaced,) which has had every component upgraded except for the CPU and mobo, so, Mark, I got you on spending the money now for a long-term system... it just takes a bit to digest a $500 CPU.
I also clarified the video and audio editing... typically what I do is transcode (which wads up my 2500K, nowadays,) but it's at the hobby level; I do it while I'm working on the PC, so I could see where more cores would help with that.
...and...
In just the very little research I've done since I started this thread, I have seen conversations on issues with the 3600 AMD chips. Long boot times... that's what I have now, and one of the reasons I'd like to upgrade. Of course, with a more stable OS, maybe I don't have to reboot so often, either. I'm assuming these problems don't plague the 3700 and 3900 chips?
No, Mark is correct... no gaming.
And, yes, I am a bit concerned about adopting to a new platform (x570.) I can assemble hardware all day long, I am not a software/programming guy.
As noted above, I see your genius on the 3900x system, it makes sense. I will have to continue my research to see if it's the right fit for me. I'm curious, however... everyone seems to be split between the 3900x and the 3600x, is there anything wrong with 3700/3800x?
I also realize this is 8 years later... whereas 4 cores used to be the standard, now we are looking at moar cores... and likely for a reason. The 2500K was such a stud of a processor... I want that in 2019 parts, if you see what I mean.
Speaking of mobos... If I'm not going ITX (and I understand the reasoning not to do so) the biggest I would want to use would be mATX, so I could reuse my Fractal Define Mini. Unfortunately, Newegg lists exactly ONE mATX x570 board... is x570 so new, or has mATX fallen out of favor???
I knew about M2 SSDs, I've never heard of PCIe 4.0. I guess I'll have to look that up. And, Larry, I think you are right... I won't be putting anything more than a lower-end EVGA video card in. I'm also curious about why a SSD would throttle from heat... I thought that was one of the beauties of SSD's... no heat?