So your lifting water 20 feet. The water is at the top of the screen and a centrifugal pump can only lift water 25 feet max. These are not ideal stats.
The 20 years is a long time for a screen to go without plugging up. I'm sure that is most of the problem. I have seen them plug up in less than three years. It's all in the water quality.
Do a Google search for Nu-Well Tablets. They are made by Johnson screen. I have had good success with them in the past (30 years plus now) but they must be used correctly or they don't work at all.
I used to put them in the well one at a time so they didn't bridge in a joint. You want then in the screen. Then you must find a water source to keep washing past them to dissolve them and push the mix out into the aquifer where most of the plugged sand is. This takes hours. Overnight is better.