I am the youngest of four siblings and was unplanned. My Mum had me when she was 43 and there are 12 years between me and the eldest (5 between me and no. 3). My Mum says that my babyhood was definitely the easiest for her - she was so relaxed, she was practically horizontal, the others were all at school full-time and when they weren't at school, they could be trusted to do everything for me apart from breastfeed . She felt no pressure to be "out there" making friends - we would see a few other Mums with children the same age as me, but otherwise, my socialisation was left to siblings and their friends and nursery when it was time for me to go.
I think my arrival kept my parents younger for longer IYSWIM and from my point of view, they were very relaxed with discipline issues having finely honed their technique over the years.
I can honestly see only a few downsides:
Back to babyhood - yes, not great when you've left it far behind, but you know it flashes by in the grand scheme of things.
Lack of freedom for you - I think you would be much more laid back about taking advantage of childcare if and when it presents itself. Chances are you could afford it more easily now too.
Family occasions/holidays more difficult - undeniably true but weighed up against that is the fact that your two girls would be invaluable helpers to you and older siblings tend to be much more "clucky" and less jealous IYSWIM.
Phew! Of course, the decision is yours (and I don't envy you having to make it) but I just wanted to put the child's POV and hopefully get across that this baby could bring an unexpected but joyful extra dimension to the lives of all your family members.
Best of luck