Do my gardening in southern Ontario, Canada. Very different climate.

However, St Pete Beach (Gulf side) is our (winter) second home. Don't work down there!

Noted there's a 'Florida Gardening' forum (GW) and some comments under 'Growing Echinacea/Coneflower in Florida', which you've very likely seen, but which don't answer your question.

Noted a comment on DG from St Augustine about them being able to take the hot Florida summer sun, being prone to fungus in that area and about no problem with reseeding; maybe the species.

Sorry it very likely doesn't help, but 'Magnus' and 'White Swan' are the most dependable cultivars here.

I'm in Michigan, so not sure what will work for my climate will work in yours - but "Magnus" and "White Swan" are tough, reliable performers for me. :0)

I would stop in or call Heathcote botanical gardens and ask them what they think. It's right there in Ft Pierce and they've got a few perennial beds and a ton of stuff that might be easier to grow than the coneflowers.

But I know what it's like when you get stuck on a plant that might not be the easiest thing for your garden. You might have luck growing it as a fall planted annual... if you can find started plants at that time locally.

Oh and you could also try McKee Bot gardens just up rte 1 in Vero Beach. They might also have some local advice.
Wish I had two botanical gardens within 20 minutes... even if they are small.

Here is a link that might be useful: heathcote bot gardens