I had attempted all that the previous evening:
Shut down every conceivable program and app other than DCS
Turned off all peripherals other than mouse to test if it was a device conflict
Confirmed firewall settings were as previous with DCS as an exception (it was, but I also tried turning it off completely, as well as antivirus off)
Reduced graphics to potato-tier settings
Rebooted modem (directly linked to the modem)
Turned off Wifi and Bluetooth
Rolled back GPU drivers to an update a week or so ago
The only things I didn't try (that I could think of - I barely know a thing about PC's) were uninstalling and re-installing DCS, and doing a full Windows re-install.
All of it failed, or actually seemed to make things worse, as I couldn't even click on menu buttons without it freezing and needing to be closed from the Task Manager - whereupon I started this thread. I reset everything to where I'd started and went to bed.
And then, lo and behold, when I got on tonight to set a baseline as a control and then start trying other solutions, the game worked FLAWLESSLY on the first go. Tried simple single-player, complex single-player with a ton of AI assets, and then multiplayer on a busy server. Ran buttah-smooth at all times. Better than it has in since an update or two ago, actually. Absolutely perfect with zero stutters or lag whatsoever after the MP server loaded everything in.
Bizarre, because I had changed absolutely nothing from where it was set last PM when I started having problems, and the game was completely unplayable.
No bloody idea what caused it, or solved it. Appeared and disappeared out of the blue. Dunno if you guys tweaked something last PM, but it works superbly now.
Appreciate the help, though.
MickV's post in SAM Site locations no longer indicated on map? was marked as the answer