I have a DD age three and a DS age 6months. It is hard going, although we don;t have to sope with reflux. In fact my brain is falling apart right now but have a couple of minor suggestions.
In addition to DH doing dd1 friendly activities on the weekend, could he also take DD2 for a period to give your DD some time alone with you?
It's very hard with the reflux situation. DD1 is probably as stressed about it as you. Can you talk to her about how she feels when the baby cries. It might help.
DD also likes being a little helper.
I find Ceebeebies a great help while bf. In fact, DD won't do ANYTHING else while DS is latched. I let it slide as it makes life much easier.
Can you get her in a car to a park or soft play where DD! can run around? Playdates with friends at their houses? My mantra is, Get Out Of The House, although you have said you find this difficult. For me, it is harder coping with two while staring at a pile of ironing and washing up.
We also could afford a cleaner once a week which makes a huge difference to my stress levels. Could you?
We do tonnes of sticking and painting - I can get her to do this while bf although DS's latch gets mightily disturbed. Might not be the best for reflux.
If your DD1 is into books there's a book called 'Silly Baby' which is good for unleashing toddlers frustration with baby.
ummm...hope one of those might be of some help... off to collapse.