Line that is flexible plastic and 20 years old tells a lot about your options.

Wonder what pressure test the line is? That tells a lot about your options also.

Wonder what you are paying to dig up multiple times to find a leak?  Yet another factor to consider.

We had a leak years ago to the barn to water cows.  We knew the old plastic line was only 80 pound test, at the time it was put in, that was considered a lot, but times had changed.  We could see water coming to the top of the ground, but it sometimes channels through the soil and comes to the surface elsewhere, not necessarily where the leak actually is, plus a small leak is hard to find when you dig to find it.

We thought our best option was to bury new line, cheapest in the long run rather than dig several times to find a leak, and then find out another leak developed somewhere else.  Labor costs today, or opportunity cost if you are providing your own labor, are too high to waste time digging to find a new leak, especially on only 500 foot of water line.

Were it me, I would bury new water line and forget the old.  It is hard for us farmers who are used to finding problems and solving them to adopt that practice, but I'll bet it would be the cheapest solution for you in the long run.