Thanks for the input, @oshizu @GlazeyLady & @Deklitch
The factory reset using the link given by @oshizu seems to have done the job! Phew! I think in future I'll perhaps try to remember to do that before starting any large project just to clear the decks, as it were.
My game stutters and freezes after several generations, especially if I have too many saves and Tray folder items (households and lots) in "The Sims 4" folder.
Whenever I start a new multi-gen challenge (dynasty, legacy, or whatever), therefore, I always rename my old The Sims 4 folder and have the game create a new one, which acts as a bit of a factory reset.
For example, I last played a casual Cottage Living game. After finishing, I renamed that folder "The Sims 4 - HoB casual" and started a new game to create a new "The Sims 4" folder.
Also, I have one folder for tournament challenges, which is currently named "The Sims 4 - vanilla" (b/c it contains no cc or script mods).
But if I play a tournament challenge, I'll rename my current "The Sims 4" folder to "The Sims 4 - BDD" then rename my vanilla folder to "The Sims 4" until I'm done.
Does that all make sense?
It's too late to do this with your current dynasty, but it's something you might think about before starting your next multi-gen challenge.