Here is what I think....
I think that by the time your new baby gets old enough to really start enjoying Disney - your oldest may have started to outgrow it. It isn't a certainty, but its a possibility. With kids spread out that much, you'll have waves of Disney.
I wouldn't buy enough points to go every year - I'd plan on every other year - and I think looking at renting is a good suggestion. Your oldest two will be able to run off on their own at Disney in a few years - which may alleviate some of the boredom with "we've been here a dozen times and it isn't as cool as Universal" (or Hawaii, or skiing, or whatever else they get into). Also, your oldest may be entering Summer activity age, and that June-July window may be closing - my son had Summer ball during that time for years, my daughter is always doing Summerstock during that period. Look at what your oldest two are starting to get into and talk to parents of teens who have those interests. And make alternative plans - my mother comes over to "housesit" and make sure he doesn't end up with a party in our house - and he stays home - because there was the horrible vacation where he got dragged along - a petulant teenager on vacation is not fun (and its too expensive to bring someone who doesn't want to leave the hotel room). Hopefully, yours will avoid this stage - some parents get to skip it.
I wouldn't worry about BCV, especially if its a financial stretch. Its big advantages are the pool and the Epcot/DHS walkability. But the pool is again, something my kids hit a sweet spot for and were over in the blink of an eye. We like the walkability (we own at BWV) from that area, but we could live without that if it was a financial stretch and I was remaking the decision.
Your ten year old is on the cusp of rapid change - and you know how quickly babies change - you are going to turn around and discover that the oldest is off in college.