I dont mean to put a downer on it, but it needs thinking through.
Firstly - are you sure you're ok about leaving DD for 3-4 days? Sounds like you are, but I was in theory and when it came to the crunch, theres no way I could have left my babies for that long at that age.
How do you think DD will feel about it? ( I know I know she's a baby but do you think it will cause her distress if you go away for that long?)
If you are going to do this, you need to get her started on a bottle SOON (and I dont mean on formula, I mean major expressing) so she's used to it. I dont mean exclusive bottle, but at least one or two feeds a day.
Solids are pretty irrelevant at 5.5 months. She'll need the same amount of milk. Are you prepared to give up bf when she's that young, because there's a chance she'll get used to the bottle and will be unwilling to accept the breast when you get back?
And also, when I left my DS overnight once at 4.5 months, my boobs were killing me. I couldnt express it enough - I left at 4am to get home so I could get DS on there to drain them!
How is DH/DP about it - are you doing everything at the mo, or is he getting 'down and dirty' so to speak. He will need training up too if he's not massively hands on. My DH would NOT have coped with a 5.5. month old overnight, especially for 3 nights.
I dont begrudge you at all for wanting to plan ahead for some fun, but it might be a little optimistic!