Nice plan -- a number of years our plan was about the same - but we bought the boat early and sailed on weekends in Biscayne Bay to get to know the boat a bit.
First boat size -- We sail a Jeanneau
DS40. When we started we wanted something between 38-42' - Knowing what we know now it would be between 40 and 44' - We have put a lot of miles under our keel
in the past 8 years and for us this is a great size.
Your idea of some coastal stuff first is a great idea and we constantly tell people to do that and include the Bahamas
as it is easy cruising but requires a bit of skill and you really get to know the boat and what you need for long distance cruising. We did the coastal/Bahamas thing for 2 years and on year 3 we came back to the Cheaspeake and spent a bunch of money
to upgrade her where we found we need stuff
By the way a boat does count as a second house for tax purposes - not sure if you entered that into your calculations.
Good luck with the plan. You are where we were about 13 years ago when we bought SoulMates and we took off 8 years ago.