First, I defer to
@eagle2250 who lives there and I agree with
@SG_67 's query.
With that said, the last time I was in FLA - which was about 8 years ago now (time to go again) - I stayed in and went around Miami and South Beach and, other than people who had jobs that called for it (hotel managers and some businessmen, for example), almost no one dressed up in any traditional sense (suits, ties, etc.).
Even going out at night to some of the nicer restaurants, many men were in shorts and t-shirts and my lightweight chinos and linen sport coat with an open-collared shirt put me in the dressiest 20% - and, again, that was in the nicer places. When we went casual, there was hardly a sport coat in the place.
I was there in March (maybe early April) and for a Northeast resident (like you), it was (what's the term?) hot as blazes, so the lighter weight (and color) the clothes the better.
As to some of your direct question. I agree, jeans, for me, are just too heavy in that type of weather, but you will see a lot of people wearing them both during the day and at night. I don't get it, but people do it.
I would suggest you bring a pair or two of your lightest weight chinos and, if one holds a crease, you will have all the dress pants you need (unless you know something very specific that you are doing that requires dress clothes). Combined with a couple of lightweight button-up collared shirt, you should be good to go. I'm not a short-sleeve button-up shirt guy, but you'll see many people wearing just that in FLA; so if you like that look, you're good to go.
As to footwear, also think informal. During the day, I wore lightweight canvas sneakers and at night lightweight driving mocs and I fit right in. I also had and pair of linen Espadrilles that were perfect for day or night (and comfy as heck). Again, other than those whose job seemed to call for it, almost no one wore tradition dress shoes, etc. The dressiest footwear I saw were leather loafers (the thin, expensive Italian kind), which many men did wear to the nicer restaurants and bars. But again, they were in the minority and you'd be fine in something less formal.
I'm just old enough, 55, that I can't go on vacation without one sport coat, but unless you know you are going somewhere where you will need it (I can't think of where that would be), then you probably don't need it. That said, I don't go to clubs (a big South Beach thing) - and never did when I was clubbing age - so if that's your thing, you should check into how people dress for those venues.
Last thought, it does rain - usually for an hour or so a day - in FLA. It will be sunny, then a rain shower will happen for an hour in the late afternoon, and then it will get sunny again on many days. Just something to consider. Neither my girlfriend nor I care that much, so we just got a bit wet (it was a good excuse to pop into a bar and let the weather clear) and let our summer clothes dry, but you should at least be aware.
Most importantly - have a great time. I've always enjoyed FLA and hope you do too.
