Not all parks are created equal. I hunt a small rural neighborhood with many ‘parks’ in it. There is also ‘park’ land around the neighborhood, all green on the map.
To make a long post short and without describing each one, they are all much different. I find the larger ones have no proximity spawns and few ‘park spawns’. I find all of my park spawns in the small neighborhood parks and the surrounding roads. I have visited a few large state parks and found slim pickings on actual park spawns also.
So try looking at smaller, neighborhood parks with a few SD around them. That is where I have my best luck on park spawns.