New iMac 2017. Same issue. Very annoying!
However, setting the mouse to "without inertia" in Accessibility as you describe fixes the jitter thanks
Interestingly following this:
"Without inertia" in Safari if you flick your finger up the Magic Mouse the page jumps down by a chunk. To scroll further you either have to run your finger up/down more of the mouse or if you flick your finger in a curve shape i.e. slightly across the mouse then up/down then the screen scrolls akin to how it did "with inertia" only with noticeably less jitter. This trick only appears to works on some webpages however - e.g. it doesn't seem to work on this support page, but it works well on BBC News, Amazon.co.uk. Doesn't work in Word
In Photos, to me the scrolling is better "without inertia" where oddly there is inertia i.e. a gradual slowing, with only a very subtle occasional jitter at the end that is nowhere near as disconcerting as the jitter in Safari "with inertia". "With inertia" in Photos the screen scrolls smoothly, slows down nicely then comes to an abrupt stop - which is more disconcerting and has less inertia than "without inertia"....
I guess you try it all and see which works better for you. Let's hope Apple fix the issue soon!