Perhaps an unpopular opinion, but I found the best place to store excess grain is at the local homebrew shop. Yes, I own a shop so you might think this is self serving but hear me out. When I didn't own a shop, I wanted to be the king of my homebrew domain and sourced several sacks of base malt, several pounds each of different specialty grains, storage containers, a mill, etc etc. I was proud of myself for "saving so much money" on bulk grain.
Two problems. Whatever grain is starting to age out, if you are so inclined to track such things, starts steering your brewing decisions. E.g. "if I don't brew a stout next, I'll never use this grain". Then you fight against that constraint and brew what you want only to dump 50 pounds of grain in the garbage.
Not everyone has a decent local shop, but if you do, think of the increase prices of by-the-pound grain purchases a fee you pay them to keep the grain fresh for you.