IMO, the only way to make money on a Galveston rental that isn't paid off is to have a large-enough home to justify high rates (say, $3K-$4K for week in the summer) and manage it yourself through VRBO or AirBnB.
I owned a home in Pirates Beach for eight years (sold in 2018). We used Sand N Sea as the management company, primarily because of their marketing strength. Sand N Sea takes 20 percent of the rental income, which is pretty standard there.
Our house was only two bedrooms, but it was a block off the beach and had a great deck and view. Still, because it was small, we could not charge much for rentals, even during the busy summer months.
We were typically booked solid from mid-June through mid-August, at spring break, and over major holidays, with a lot of weekend rentals throughout the year.
However ...
We did not turn a profit in any of the eight years we owned the home, and after expenses, it was even difficult most years to cover taxes, insurance and maintenance with revenue.
In addition, increased competition due to new builds in Galveston actually pushed rental prices downward beginning in 2015-2016, which continued through the time we owned the home. We could see the writing on the wall -- taxes and flood insurance were getting more expensive and rental revenue continued to fall even though bookings were consistent.
I have a relative who owned a larger home in Pirates Beach, a four-bedroom. He too lost money every year, despite a busy rental calendar. He used Gary Greene for his rentals.
I also have a friend who owns a relatively new home on the West End. Her family manages everything themselves -- booking, client communication, etc. -- and they've hired local vendors for cleaning and maintenance. It's a very expensive house with high rates, but they were not breaking even with a management company.