Do you hold R2+Cross (ps4) to climb, or just to get started?
You use it to climb. But just to get started! To keep climbing you can just press R2 or use the Left Sticker to climb up. This way to climb jump you press X.
I generally prefer to hold R2 because it has been a habit of mine.
So you're running with R2 to a wall. To start climbing you go R2+X. To keep on climbing up you cab hold just R2 or climb normally (Left Sticker up of course).
Do you have to hold Circle when descending or just press it for each
drop you want?
If you want a continued descent you must press R2+Circle. If you want to stop the descent you just stop pressing the Circle and you are no longer in descent.
[EDIT]
Keep in mind that the direction you're holding in the Left Sticker matters for Climbing!
I've had situation where I was holding up on the Left Sticker and pressing R2+Circle and I didn't descend.
There are still some bugs to it but mainly this is how it works. Hope I Helped!