Mike Perkins was thinking very hard :
Having never used these before and having to de-rust a large steel plate with
modest pits, what is the most suitable grit size I should use?

This is for a 4.5" / 115mm angle grinder.

I also got the impression the finer grit sized wheels last longer?

Can anyone assist with pros and cons?

Electrolysis rust removal works rather well and only removes the rust.