Realtime Preview already takes into account browsers' prefers-reduced-motion setting, and enters manual mode when it's reduce (that was done in T314787).
It would be good to be able to turn off automatic reload manually though, so that the preview pane isn't constantly reloading (either to reduce annoyance, or while someone is doing a bunch of editing that they're not ready to preview).
The UI design for this is the main unknown.
- This is a follow-up to T314787#8175701
- Also requested at https://meta.wikimedia.org/wiki/Talk:Community_Wishlist_Survey_2021/Real_Time_Preview_for_Wikitext#Toggles,_please