In the books you've got spells that's marked by loud bangs, subtle breezes, by their invisible instantaneous effects or flashy bright lights.

In the movies nearly everything is a bright beam of colorful light that makes you think "why aren't anyone using guns instead? It'd be way more effective."

Does anyone have examples from canon, or a list of spells that are of the subtle variety?

Maria Edgecombe getting Memory Charmed by Kingsley - that was a breeze.
Malfoy getting de-ferreted by Mcgonnagal - that was a loud bang.

How do you 'personally write/or read' the visuals of spells in Fanfics?

 

The films are incredibly inconsistent in their portrayal of spells. Sometimes its a flash, sometimes its weird lightning stuff, and sometimes its simply "shout spell, effect happens" with no visuals.

I prefer the idea of a huge variety of different visuals, with most spells appearing slightly different. The stunning charm and the reductor curse both are red bolts. I would take the approach of describing them differently, either in terms of shade or some other factor. Maybe one is a straight flying bolt of solid red light, and the other is more like rippling waves of red light. Or one is crimson and the other is a dark blood red. Something like that. But I'd definitely make use of the sensation and audio effects as well.

 

The one thing about spells in the movies is their speed and physicality - from the OotP movie onwards, there's a real sense that a duel is extremely dangerous. I think my favourite scene from the movies, on that front, is the Snatchers chase scene in DH Pt. 1.

But yes, nowhere near enough variety in the movies.