I’ve noticed that my black SLX 10 speed mech is showing wear on the inside of the inner cage above the bottom jockey wheel, is this normal? (I.e the chain has rubbed a lot of paint off)

probably not indexed right. Check the alignment of the jockey wheel with the cassette sprockets in each gear. Also check the hanger for bends.

Thread on here recently about lower jockey wheels shifting on their bearings, is yours sitting centrally?

yes perfectly normal, chain can move a little on the jockey wheel, jockey wheel wears, chain bounces around a little. etc

Thanks – will check the jockey wheel. Gears are indexed properly and no issues when shifting