SBI is a great mail service, IMO. There are two ways you can use it though:
1) Maintain your domicile in another state, and just use them as a mailing service.
2) Change your domicile to Florida, and use them as your address/mailing service.
Sounds like you want to do #2. Something to think about is that you need to change enough things for it to be legit. I don't know that there is any cut-and-dried line, but the more the better. Basically, you don't want it to look like (or be) a sham and you really live somewhere else.
So, things like health insurance, banking, car titles and registrations, voter registration. These would be best in Florida, and not great to keep in your previous state (as if you really hadn't moved to Florida but were just trying to take advantage the concept).
Probably no-one moves every last tie, and I don't necessarily think that's required, but the more the better, from the perspective of making it genuine.
A couple of possible downsides:
1) If you have a commercial driver's license you will have to give it up unless you have a street address you can use (say a friend or relative). You can't keep it with the SBI address.
2) Some banking or similar may balk at that address (others may not, especially if they are accounts you already have vs. opening new).
3) Boat or car insurance may be more due to hurricanes, etc. (but maybe not more than CT, which at least in my mind is an expensive place to live?).
There are cheaper states than Florida in terms of boat sales tax, but it's also not anywhere near the most expensive. Florida is used to people living there part time, and SBI is established in the community, which helps. I can't overstate how great a good mailing service is. Just one example: A couple of years ago the Clay County elections people decided that "outsiders" (i.e. those domiciled say at SBI) shouldn't be able to vote in Clay County (funny thing is, they were obviously one "color" of party and assumed most SBIers would be the opposite but I actually think they likely had it backwards).
This wouldn't have been such a big deal (local elections) but the way the USA works is that unless you are registered to vote locally, you can't vote in national elections either. That would put out a lot of military, traveling nurses, and of course others such as boaters. Suddenly you'd be disenfranchised. Luckily SBI was organized and went to bat for their customers and that was resolved.
Some other considerations (which you may already have taken into account): Do you use health insurance? If so, then wherever you settle, consider whether the available plans make it a hassle to get care outside your home community. Insurance (already mentioned). Then things like personal property taxes (those can quickly make a "low tax" state look darned expensive when you have things like boats and cars and RV's). In other words, just look at the big picture for you individually, because a "no tax" state might be best or might not end up being great, just depending.
One "don't," IMO, would be to have a Florida domicile, a boat documented in another state (especially a no-sales-tax one), and then boat in Florida. They look for that.
For travelers, Florida, Texas, and South Dakota are the usual "go to's" and make it fairly painless. But they may not be the best choice just depending on your individual situation (for example, SD is pretty sparse on health insurance plans if you are pre-medicare).
SBI is top notch though! Even if you "only" use them as a mailing service but domicile elsewhere.