I used to only mix up a partial bag too. Now I just mix the whole thing up and put it in a wine bag in a box. It’s air tight and lasts a very long time. When it starts to get low, mix up another couple gallons and put it in the bag.
That being said, I don’t do much D76 as of late. Largely switched over to replenished XTOL. I make up the 5 liters, and put it in a wine bag, and keep a 1 liter Amber glass bottle as my working solution. For every roll of 135-36 I put 70ml from the wine bag in the bottle then top the bottle off with what I just developed with. For 135-24 it’s 56ml per roll. When the wine bag solution gets low, mix up another 5L and put it in the bag. Piece of cake.
I can certainly understand why some people don’t want to replenish, and they should do what works for them, however in my experience, XTOL is one of the easiest systems to replenish and Kodak’s datasheet for it is excellent.