Underseason your food, taste it, then reseason to what you think tastes good. THEN RETASTE IT AGAIN. There's a reason there aren't salt and pepper shakers on higher end restaurants. The plate put on your table is what it SHOULD taste like.
You probably aren’t mixing certain things as much as you should be, like creaming butter and sugar isn’t just combining them, you need to beat them until they’re fluffy. The same goes for adding eggs...you’ll get better cookies and brownies if you beat again until fluffy when adding eggs to your creamed butter and sugar. However, you don’t want to over-mix once you add the flour, just mix until no more dry flour is visible. Over-mixing the flour can make your end product tough.
I'm happy you got that passive/aggressive thought off your mind.. Cook on, big fella.
#6 Grocery store chills some vegetables but not others, I do what they do.
#9 Mix sauces similar to mixing paint; alternate top to bottom and outside edge inward.
#12 An accurate meat thermometer is a crucial tool. Learned my lesson after I overcooked a good piece of meat.