The first thing you need to do is reduce the length of the stock, as the 14" LOP is too long for most folks. A LOP which is too short is easier to manage than one that is too long, but a proper fitting stock will make go a long way towards making recoil tolerable and provides a good opportunity to upgrade the recoil pad.
Next is your shooting technique. Stand with your feet should width apart quartering the target. With the gun mounted properly into your shoulder, bend your forward knee and lean slightly forward. This allows the recoil impulse to travel across your body and down your back leg rather than absorbing the entire force into your shoulder and chest.
Some 12 ga ammo is particularly objectionable. Stay clear of 3" and 3.5" shells. If you have trouble with 2.75" high velocity loads shoot low brass shells. Low brass shells are fine for small game and upland birds to say nothing of a day at the skeet range where a large quantity of shells can be consumed over the day. If you want/need to shoot buckshot and slugs choose managed recoil ammo or handload the equivalent.
I'm not particularly in favor of increasing the weight of a gun as this usually screws up the balance of the gun; the easiest place to add weight is in the butt stock, but the gun points best if it is slightly muzzle heavy. If additional recoil reduction is necessary, I would look at a different gun. A 500 with its aluminum receiver is pretty light, particularly if mated to a plastic stock. All things being equal, a 20 ga kicks less than a 12, although this depends on the specific ammunition and the guns in question. A 20 is typically smaller and lighter than a 12 and a 3" 20 is similar to a low brass 12. A gas operated semi automatic action tend to recoil softer than any other, but an inertia driven action has no recoil reducing benefit. However, if the gas gun is lighter than your 500 pump, it will still jump around pretty good.