You live in South Florida and have had some of the galvanized pipes replaced and there is no basement (believe it or not, I’ve seen two basements in Coconut Grove and Coral Gables) and have not crawl space (you didn’t say this but I suspect that is also what you meant)?
Being also from South Florida, I suspect you live in old Hollywood, old Ft. Lauderdale, or Miami areas where they built with slabs and galvanized pipe before realizing they shouldn’t do that.
As you’ve torn the wall down behind the kitchen sink, my first suggestion would be to remove the riser to the kitchen sink faucet and the cut off valve screwed onto the galvanized coming up out of the slab (make sure you turn the water off outside and leave an outside hose bibb faucet open to prevent pressure build up), then look at the inside of the exposed galvanized pipe. I suspect that you will find it is clogged.
If so, you will need to have some re-plumbing done.