Will external GPUs never work? As in, physically impossible? That'd be my main purpose.

Physically impossible, no, but external GPUs definitely work right now with Intel processors but even they have their limitations related to usability moreso than technical issues. I have one now with a 1080 Ti off of an Intel NUC and not being able to see my BIOS without switching to the iGPU can be cumbersome when troubleshooting. Once in a while when I reboot (this is very frequent actually because my eGPU's PSU's power limiter cuts out if I forget to throttle the power usage) the GPU doesn't come back unless I hard cycle a couple times.

The annoyances are such that I'm waiting for a 3900X to be available in-store so I can build my full workstation finally after sitting on a NUC + eGPU setup for over a year.

I don't want to buy Intel CPUs, nor Nvidia GPUs. Exception being an Nvidia Shield.

You'll be going for 3900X + X570 Taichi? Or another board? Ideally I'd also want ECC RAM.

I got a 3900X and the Asus X570 workstation board to make sure I'd be able to support ECC RAM within basically a couple hours after I made that post. My needs are video encoding and virtualization setups needing lots of cores so this makes a lot more sense for me than any comparable Intel setup for my money. I'm slightly cheaping out by not going with ECC RAM but in the era of catering to overclocking gamers over professionals I'd rather pay more for a workstation board.