Definitely don't worry about glamming out. After a few Celebrity cruises, we cruised on Princess for our 20th anniversary. I packed our normal cruise clothes. Talk about embarrasing. DH & I were waaaayyyyyy overdressed. We stuck out like a sore thumb.

Everyone was commenting on my dresses. I guess some people would have loved the positive attention, but I'm not an attention seeker. I actually dreaded going out for the second formal night. Fortunately, the third formal night was Halloween, so we wore our costumes.You'll be fine wearing your sandals & the same dress twice. From what I've read, a lot of women wear the same dress both nights. Some change accessories for the second formal, but I'm sure many don't. No doubt you'll be more comfortable than I was.
If you plan to retire to Miami, be very careful about the area you choose. There are some really nice areas around the city. Unfortunately, they're also very expensive. We live across the line in Broward County. I don't think I've seen where you currently live. If you live up north, house prices may not shock you like they did us. They're outrageous for a southern state. They're at least double what a similar home in Tampa costs. Needless to say, we had to downsize to stay near (not on) the beach & close to both Miami & Ft. Lauderdale.
Traffic is horrendous in both counties. It wouldn't be too bad, if you're retired though. For this reason, were considering moving & renting our home, then returning, when we retire.
Miami/Dade & Broward Counties definitely have their attributes too. The beaches are nice, the water is beautiful, the weather is great & between the two counties you have every type of entertainment you could ever want.