Even better!
Yes, convert that lawn! Two plants worth considering are Prunus persica 'Kernechter vom Vorgebirge' and Amelanchier × lamarckii. I've heard great things about them, but they were growing in SW Germany, so do your own research first. What do you use for a ground cover? Clover can be a valuable ally in the fight against the grass.
You have a ton of potential in South Florida! (Until sea level rise floods everything, of course.) Will you add more fruit trees? The nurseries in your area have some amazing options. Which mangos and avocados do you grow? I'm curious about the quality of 'Monroe' and 'Oro Negro' avocados.
It's strange what's happening with your avocados. Do you know if the bloom timing of avocados in your area has changed at all? If they were previously getting pollinated by trees that now bloom at different times, then that could explain the lack of fruits.
If it's a choice between banana and grass, I recommend banana 100%. Pine Island and Excalibur both sold Dwarf Namwah last I checked, and that should be very productive. Excalibur also sells FHIA-18, which doesn't taste so much like banana. I recently posted about it here, though the linked PDF is in spanish.
Sounds like you've got a great thing going! Maintaining fertility by mulching with cut vegetation and composting "waste" is really important. How big is your fruit forest? Do you plan to diversify further and fill up the field? How small do you plan to keep the trees? I've found that pruning tall trees with a pole saw is really tedious and exhausting. Do you have a particular method that's easier?
Not surprising. I remember reading that (some of) the ancestors of Home sapiens sapiens in East Africa first started regularly using fire about 400,000 years ago... to cook starchy tubers. Apparently that population didn't start eating animals for another ~300,000 years, and others didn't even leave the forest and start using fire until ~250,000 years ago or later. History is not an ethical guide in any case...
Is there no chestnut blight in your area?