As I look to grow, I’m considering joining a coaching/mentorship program like Partner Driven or something similar.

Stable demand, in retiree market - yes, but also in several other segments: professional working remotely, new Americans. 

There's just not a big demand here for STRs.

For first-time investors, neighborhoods like Riverside, Murray Hill, and North Jacksonville could be good places to start, offering strong rental demand and potential for growing neighborhoods.

Also, What do you consider growing to a moderate size?

While RAD also concentrates on areas of Texas, Idaho, California, and Florida for its growing portfolio of rental properties, according to the January offering circular, Philadelphia has been its primary locus of activity.

I like the idea of getting additional liability protection on both the property + a commercial umbrella policy as we are planning on growing our portfolio.

Like you said, you don't want to grow super fast so take your time and watch the mortgage markets.

Unfortunately, I see a growing trend of a lot of service members renting their homes at break even rates or at losses.

There is ALWAYS a demand for ocean-side properties in FL, and these are not only retirees.