Yes, you really don't need to test the borates. Just add the proper amount after each refill. It doesn't break down so should last through until the next water change and its exact level isn't important. You just want enough for decent pH buffering and 50 ppm is a good amount, but even 30 ppm will help and 70 ppm is not dangerous.
Now for a pool that's a different matter where testing is important because you generally don't replace all the water so need to know how much got diluted so one can bring it back up, usually once a season.