I’m only 19, but I have almost 5 years of forearm/grip training experience. I started working only on my forearms because of an injury to my wrist from hitting the heavy bag without wraps, while pissed off. Stupid, I know.
While you said that you did not want to get certified on grippers, I would still get a No1 and No2 CoC. Some people will tell you that they are useless and only help you close grippers; I have to wonder if they ever used them. Grippers won’t directly help much on thick bars, or with open hand strength, but I can close my No2; I can also crush almost anyones hand in a handshake.
To tear phonebooks you need mostly wrist and thumb strength. Wrist curls and extensions are a good start, but leverage work with a hammer will give you the most bang for your buck. Ironminds Heavy Hammer II is nice but expensive, I made one from some tubing and an Olympic weight sleeve adapter.
For thumb strength, pinch grip is good, taking plates and holding them for time. Also plate curls and plate wrist curls. The thick grip leverage will also fry your thumb.
Rolling frying pans, to be honest, I don’t know much about. But I imagine a thick grip roller would work well. Buy a length of 2 or 2 1/2 inch pipe and attach a strap in the center. Straps roll more evenly than rope.
Most grip exercises that are suggested always seem to be static movements, but most feats of strength are dynamic.
The coolest grip toy I made is a softball with a loading pin. I did not come up with this, but I can’t remember where I read it. Take a softball and pound a nail into it to make a pilot hole. Don’t go all the way through! Apply some epoxy to a small eye bolt and screw it in the hole (please make your own joke). Get a carabiner and a loading pin and you have just about the most versatile tool out there. If you grasp wide, it works your thumb and pinky hard. If you keep your finger together, it is like a pinch block, except as your grip tires and you squeeze harder, the shape of the ball makes it want to shoot out of your hand. You can even try to rotate the ball with side to side movements of your fingers.
If you are really bored and know how to splice three strand rope, you can back splice a 3/4" or 1" rope into a thick rope. I took a 3/4" manilla rope about 12’ long and back spliced each end about 2 feet. It is about 2 inches or slightly smaller, and it can be used for tug of war, rope chins, rope deadlifts etc. Flea markets are good places for old dirty cheap rope. In addition, splicing an old, stiff rope is a great finger strengthener by itself.
Have fun with grip training, it is addictive.