Nitrate level at 20ppm is not a problem. I would consider 5 to 20 to be great. Dangerous levels don’t start until you get 60/80. Ammonia and nitrite should always be zero but the result of that should be some amount of nitrates.
Sounds like you might have a bit of a misunderstanding of "parameter shock".
Ammonia and Nitrite should both be 0, and nitrate should be under 20, in most tanks. However, doing a large WC to lower any of these values will NOT result in any shock, IMO. It's better to have a lower nitrate level suddenly than to have high nitrates that are slowly being lowered.
Now, PH is something a bit different. You want PH to be as stable as possible. Jumps in PH will cause pretty bad stress.