What to use to remove volume button flex cable easily

I recommend using heat (heat gun, heating pad, iOpener, or hair dryer if it's all you have) to soften the adhesive.

When it is soft, use the tip of a razor blade to lift the very edge, and then use a spudger to lift and separate.

Only insert just the tip of the blade and switch to the spudger asap because, as you've already found out, the cable is very delicate.

I've taken a couple photos of an A03s replacement screen I have in stock. The red arrows are to show you where I insert my blade to lift and gain access with the spudger. If you try with just the spudger, unless it is brand new with a perfect sharp tip, it will not be fine enough to get under the ribbon cable and you will end up squishing it and risking further damage.

The red arrows point to where to start to pry. There are indents in the metal chassis that should give you a good access point. If at any point the adhesive holds too tight do not pull or you will rip the ribbon cable. Stop and apply more heat and try again. It is a small cable and doesn't take but a minute when you get a feel for it, but be careful and take your time the first time.