Rubbery Chicken Legs (Blahhh)

I put a little olive oil in a screaming hot cast iron skillet, sear the chicken on both sides, then put a cover on the skillet (chicken skin side up) and pop it in the oven until the meat is just about done. Uncover for the last five to ten minutes to crisp up the skin.

You need to roast chicken legs, not bake them on low. Cook at 425.

My mother's recipe is not the healthiest...

Line the baking sheet with foil.
Melt a stick of margarine then dredge chicken legs in it then seasoned flour (flour, paprika, salt, pepper, celery salt, onion powder, and garlic salt)
Place chicken on sheet then drizzle with any remaining margarine.
Bake 375 for 1 hour

Because of all of the connective tissue in the legs they benefit from low temps for a long time, so bake at 300 or 275 a little longer and yes, cover them for most of the time then remove the cover to crisp them up in the last 10 minutes or so. Also, braising is preferred to a dry bake method so add a little liquid to your baking dish.

I've had the same issue, and despite my meat thermostat saying they were done, there was pink meat and pink juice coming out. I ended up sticking them for a few minutes in the microwave and they seemed to end up less rubbery with more cooking. Maybe that is your issue too, they are undercooked. The outside is brown and crisp, but not the inside. A friend of mine told me the way to avoid the outside getting brown and crisp and the broth and meat getting dry from overcooking to avoid the rubbery texture was to cover the pan. That way you don't end up with burned but raw meat. He used to work for a restaurant as a line cook, so maybe he has a point. It definitely isn't your utensils or seasoning, since many people using all kinds of seasonings seem to report the same thing.

Undercooked chicken is chewy.

Skip the EVOO (use dry seasoning) and cook them until the edge of skin is golden brown and starts to pull away from the bone. At 350 it will probably take at least 45 minutes.

I think maybe you're not cooking them long enough? I would cover with foil for most of the time and then take the foil off for the last ten minutes. I don't think it has anything to do with the brand of chicken. I usually cook my bone-in chicken at 400. Make sure you're using enough olive oil for the skin to get crispy. I also will dot butter on top of the chicken before putting it in the oven, that helps and tastes delicious. If your oven has a convection setting I would also do that.

I don't use a timer anymore when I bake food in the oven. I bought a thermometer. Look online and you can see what temp food should be. When I would make meatloaf, i used to cut it in half to see if it was cooked or not. The thermometer is the best thing I bought for the kitchen. Poke your meat with it and see where its at.

Higher temp, 400-425 and longer, I do 20 mins, flip, then another 20-30mins.

Shake N Bake always works well for me.

I think you can even try keeping the rest of your recipe as is if you like it, but leave them in oven for an hour. (You wrote "25-30 mins, or longer if needed" - longer is needed.)

333 convection. Salt and pepper. Nothing else. Cook for 45-60, till they look golden and crispy. You are cooking them too high and not long enough.

When I turn a whole chicken into soup I simmer it for about 5 hours... At hour 3 it will pass the doneness test but will be slightly tough or rubbery. After 2 more hours of simmering it's tender and juicy. Tears apart easily to be bite sized chunks in the soup.
My momma always said cook it long and slow on low. I would try a crock pot on low. Cooking all day.
( The chickens that I turn to soup are old layers that stopped laying. it's said that old hens and roosters are often tough and rubbery. But simmering 5 or more hours seems to render tender meat, crock pot cooking also leaves me with a nice tender chicken to eat)