Haulover Beach, Fl
We only ever went in February (and on a Tuesday to boot) and the parking lots and beach never filled completely. But in June, on a weekend, I'd plan on being there by 10am. On Monday and Tuesday you might be able to get there later and not have to worry about parking farther out and then finding a good spot on the beach.
If you need parking then get there as early as possible but if you are taking transit you have a lot more time to play with and can really arrive anytime. It's a big beach and you'll find a spot somewhere. There is some useful information on the South Florida Free Beaches website: https://sffb.com/haulover.php
If you have some time and are interested in a drive why not check out Blind Creek Beach as well? https://blindcreekbeach.net/