I kind of feel the same. We really wanted two kids, but after going through our (relatively short 1.5-year) IF journey, we went back and forth if we wanted to try again. But, I'm 34 and time isn't on our side so we thought about how we'd feel in 5 years if we hadn't at least tried.
Basically we decided that we want to at least try to give DD a sibling. The journey to get there will be worth it, once again, if we succeed. If not, we will be crushed but know that we are super lucky to have DD.
We will stop at IUIs; how many, we haven't decided yet. We have a vague timeline, so I definitely think that's a good idea.