Most of the services you recommended disabling, I already had disabled. 👍🏻
But I implemented most of your other suggestions, inc. removing the unnecessary system software. Most of this, I had done with the 10586 I had locked down years ago, but that was back when I had a lot more free time to do the research. I hadn't expected to do any of it w/Win10 LTSC IoT, but it obviously became a necessity w/all this ridiculous lag. (I still can't believe it; I've never had such problems with Win10 before!)
Anyway!
After doing all of that stuff, I've been farting around w/my computer again and it does seem to now be running quite smoother. Not sure which of your suggestions eased it, but I hope it's now going to be a lot easier to get some work done on this machine. I'll report back if it creeps in again, but at least for now it doesn't seem to be a "placebo effect" - the lag was so frequent, I'd have noticed it dozens of times by now.
So, super-duper thanks for taking the time to share all of those tips!