Re: Is this Legal?
If you call yourself a liveaboard and declare yourself as a liveaboard in florida the local municipality can limit your anchoring time and tell you to move on.
However if you do not declare yourself as a liveaboard and just happen to stay onboard every day and night then their authority to move you along is limited to harrassing you at any hour they wish as well as boarding your vessel under the guise of safety and sanitary reasons for your own safety(of course).
THe general appearance of your craft as well as your jovial deameanor when you first encounter the water authorities will determine how pleasant your time at anchor in that vicinity will be.
you mentioned there were others anchoring for extended periods in your vicinity ? I would ask around and test the waters (so to speak) before making a final decision on the relocation.
Also I would consider keeping a log of your pump outs (unless you have a composting head) as well as thinking through the availability of fresh water and other niceties the marina presumably currently supplies for the $375 a month (not bad actually for a liveaboard, and I'm ... quite frugal ).
Finally, I'd invest in 3 large anchors and plenty of chain , you'll sleep better and so will everyone in your lee.
Boat safe,have fun, or what's the point.