I have made some money off the investment, but not to a spectacular degree.

The map alone is spectacular.

They're big and spectacular, but it's not like classic little city roaches where you see 100 scatter when you turn on the light, I eliminated those when we bought 20 years ago. 

I can't speak as much to the Southern Maine market, however speaking to the market in the New Hampshire seacoast, it varies wildly depending on how far you travel away from the whole 18 miles that make up the NH coastline. 

Nothing spectacular, but also nothing to shake a stick at for a totally passive investment. 

@Matthew Wright Most communities resort type (with Single Family Homes) that allow for STR are gated and have spectacular clubhouses, golf courses, bars,  restaurants, tennis courts, etc.

Spectacular!

If your tenants are spectacular and you wish to keep them, might you consider a gradual increase over a period of 3-4 years.

With a value of 60K and rent at $600 the numbers are not spectacular.

The cashflow isn't spectacular, but if you are budgeting $210/month towards reserves and still coming out with $100/month cashflow it may work for you.