If you reduce the amount of yeast in your mixture, but keep the same amount of sugar theroetically your co2 production should be halved - I am in the process of testing this theory in my tank right now. usually I use 2 cups sugar 1/2tsp yeast but since I only had 1.5 cups sugar left! I used 1/4 teaspoon yeast. So far so good
your PH drops too fast cause too much co2? Just add more plants. they'll use it.
I wouldn't try putting a tap or valve on the DIY bottle. Kaboom!
Also cooling the water in the yeast bottle slows production. Want more production? put it on top of your light. when the light goes off, the yeast cools down, slowing production
if all else fails you can turn on an airstone at night, or for an hour during the day to blow off excess co2. Fish can take a lot of co2 (as far as I know around 100ppm) and I wouldn't worry about the pH. mine is around 6. the fish love it
hope this helps.