It sounds like your screen and gravel are too big. The gravel needs to be small enough to make a filter pack and keep the sand from getting through, and the screen needs to be small enough to keep the gravel out of the casing. We use casing perforated with .035” slots. Some people use .020”, and screens can have even smaller openings. The “gravel” we use looks more like sand, because it is filtering the natural sand and keeping it out of the casing.
If the driller used the right size screen and gravel, then maybe a little more development is needed. Air jetting and surge blocks can be used to force water through the screen. This washes out the sand and lets the gravel pack settle in the right places. Sometimes the gravel gets bridged off and is not covering the entire screen. There is plenty of gravel piled up to 55’ from the surface that can settle down where it is needed. Just need to stir things up with air jetting, surge blocks, or maybe just a bailer to get the gravel in the right place.
Because the well seems to be such a strong producer, which is good, you may never get it pumped clean with a 20 GPM pump. From the data you have it sounds like the driller is fairly knowledgeable. I’ll bet he can help you out, just hard to do this kind of time consuming stuff during the busy time of year.