Re: thinking of adding a bottle...
I don't get as much when I pump in the evening, either. That's pretty normal. I have found, though, that my body knows my regular pumping times and I get pretty engorged right around then if for some reason I'm not able to pump. You might give it a try and see if it helps.
Catherine, tandem nursing mama to Preschooler Girl 9/08 and Toddler Boy 3/11
Breastfeeding for 4 years and counting!