Like any service business with plenty of competition, you'll need to have an edge.
I recently had to hire someone for a medium-sized job. It was a job that was too big for a lot of "just lawn" guys, but too small for "tree guys". I ended up finding someone, and he did a good job, but it was a hassle getting him out to the site (he didn't really know his way around the city, etc.).
So, from my layman's view, it seems we are lacking in lawncare people with some flexibility, someone who will do more than just the regular lawn stuff.
I have something else to add:
I have lived in St. Augustine and I now live in Jacksonville. St. Augustine currently has a population of approximately 200,000 people. Jacksonville's population is appx 1 million people (more or less...depending on who is counting

). Your potential larger pool of clients is obviously going to be in Jacksonville.
Before you think Jacksonville is saturated, keep in mind how large and spread out the city is - lawncare people usually specialize in one small section of the city (Westside lawcare guys don't do lawns on the Southside, etc.). Also, St. John's county's population is spread out - you're likely to spend more time traveling from one job to another in St. Augustine than you would have to in Jacksonville.
Some aspects of cost of living are slightly higher in St. Augustine than in Jacksonville as well (traditionally, St. Aug has been a retiree's town, still is in a lot of ways).
At the very least, initially, you might want to serve part of Jax in case you don't get enough clients in St. Aug.
Above all, I would find your edge - find a way to differentiate yourself from the others - I really think that can help you overcome the competition.
Best of luck, you're moving to a beautiful area

!