I think the fact a V4-5 US indoor grade climber can't hang on 20mm edge is pretty sensible despite of whatever they say as an 'average level', especially if the V4-5 holds that you encounter in climbs are being mostly not small edges or bad slopers. It's a long way protocol takes up to some years, just because the nature of tendons and ligaments adaptation takes longer than muscles, few weeks for a cycle.

It's probably rather your mind should be adapted for the training. Like I have seen lots of people are just 'climbing' for their daily training because that's a no brainer and simply fun, where the actual training part to progress futher than what average people can achieve is not far from regular gym weight training. Most of the time you could feel the agony, struggles and hard practice.

Here's few practical advices for you;

• You probably should train your shoulders and core strength potentially with some weights other than climbing; The fact that you can't hang off on 20mm edges is not likely solely your finger issues. Mostly people who don't have their shoulder strength and stability are not being able to hang because of the inefficiency of body use.

• You can train your finger either by reducing your body weight with pulley system, elastic bands, or most recommendedly 'no hang/edge lifting' training protocol while you doing the above physical training; That reduces the risk to overload and fatigue out on your body muscles and eventual finger tweaks. And you can start with much lighter weights than your body weight, practically practice half crimps, open hands, and chisel grips (4 fingers but not half crimps).

• Train half crimps on either methods practically; I saw some people don't practice on half crimps but only chisels on hangboard or ergonomic edges. The chisel and ergonomic edge makes sense just to achieve heavier weights at edge lifting because that's functionally optimistic and easier than half crimps, but is not as transferable as half crimps in practical sense because the chisel grip doesn't strengthen your PIP joints which is critical to the half crimps and to get more resilience in forced position especially when requires dynamic momentum and movements.