Wait! One more thing!
So, as it turned out, DVDStyler was still choppy. I didn't notice because I played back in VLC Player and didn't test on regular, old Windows Media Player. So I only discovered it, slightly embarrassingly, when showing someone else to get his opinion. Subsequently testing on a Mac and two PCs, we found that Quicktime and Windows Media Player showed a similar jittery effect as what was happening before, only all throughout the video (not just at the fades) and more subtle so it was hard to detect at first. You just feel uneasy and motion-sick watching it and only realise after awhile that it's sort of speeding up and slowing down slightly all through the playback.
So here's the lesson; Test, test, test!
And here's an updated solution.
After experimenting with various combinations of bitrates and audio bitrates in DVDStyler, I downloaded and tried yet another freeware authoring software, this one called 'Free Videos to DVD'.
Free Videos to DVD is a tiny program with no features and no customisation. But, as it turned out, the files it authored played back smoothly and beautifully! So my workflow became, run my original MP4 files through Free Videos to DVD to generate those smooth VOB files.
Then run those VOB files through DVDStyler to add menus and add chapters, being sure to check the box in the video properties, "do not remultiplex/transcode," which is only clickable for VOB files.
Let DVDStyler burn the final disc.
I'm not sure but my suspicion is it was still that "muxing" (aka. remultiplexing) that was causing the choppy / jittery / jerky playback.
So Free Videos to DVD was (I assume) better at muxing than DVDStyler and DVD Flick, which is why turning off DVDStyler's multiplex/transcode checkbox on Free Videos to DVD's generated VOB files worked.
Of course, I'm not going to settle for this until testing, testing, testing!
Leaving here in a minute to try on a friend's TV / DVD Player.
I hope that wasn't too wordy to understand.
And I hope it helps someone else googling for a solution to a similar, baffling problem as I'd been having.