Traffic out of MIA will be bad, but as you said before, once you are out of there, it should be fine. I would take the Dolphin Expressway (836) all the way out to the Turnpike extension. Be aware traffic on the 836 will suck until you get past the Palmetto, if not a bit beyond. But it will also suck in the morning.
You have the choice of two ways to get to the keys, Card Sound is usually a bit less crowded, but I like just taking US1. Perhaps one way down, the other back. Did you stop at Alabama Jack's? Great little stop, even now. Fridays going down and Sundays coming back can be bad. Would avoid those days if you can. One wreck can slow you down for hours.
In Islamorada, did you stay on the ocean side or the bay side? Sunsets are typically seen on the bay side.
https://www.matecumberesort.net/
For the bay side, what about the Kon Tiki? (I have not stayed there in over a decade, but it looks like what you are searching for.
Kon Tiki Resort ? Kick back and relax in Islamorada Florida
The ocean side ones have mostly been renovated and are higher end. If you stayed on the ocean side, was it Holiday Isle? If so, long since gone. Although Matecumbe Resort might fit your bill as well. Again, haven't stayed there in over a decade.
For sunsets there, you still can't beat Lorelei's. Still good food, drinks, music and amazing sunsets. Robbie's is still a good place to eat and feed the tarpon.