That is a solid photo album, awesome build!
If the the “pause” right after starting is the deal breaker, some variants of firmware allow for a lag for a spindle to come up to speed after starting or speed changes. If using something where cut is instantaneous (like a laser, or I assume a plasma), you can disable that delay.
In GRBL 1.1, you choose “laser mode” by sending a $32=1 and the delay is removed. I’ve seen the options in Marlin, but as I’m not using Marlin on my laser cutter I haven’t dug into it as deeply there.
Of course, you still need to deal with the interference issue. I’d start with shielded cables grounded/earthed at one end only and some sort of faraday cage (metal enclosure?) around the processor, also grounded.
Awesome! I’m really look forward to your post.