Puerto Rico is a place I would not recommend for many reasons. It's wonderful to visit there for a couple of weeks. However things get complicated really quick, even if you try to avoid it when living there. If I may let's go down your list to see if it qualifies.
1) good hospitals - Negative - it's a struggle to find decent care in a clean hospital.
2) low taxes on capital gains (stock exchange/dividends) - This is a very difficult question to answer. Because of the laws and in between political status of the island you will need a good lawyer and accountant to help you get a clear answer if this would be a benefit in your particular case or not.
3 ) Citizenship by birth - It's a US territory and there is no such thing as Puerto Rican Citizenship. So for all legal intents and purposes, it is the same as being born in any US state.
4) 75,000 USD per year net must be enough for rent, living, health, etc (see 2.) It depends what standard of living you are expecting. On the par, from my experience, the standard of living in Puerto Rico is LOWER than what you would expect from a similar income on the mainland. There are exceptions, but be expected to pay a premium price if you want to live in a decent house and nice neighborhood.
5) good schools will be relevant in 6 years, therefore unimportant as for now - Private schools would be your only choice for a decent education. And even then you would have to carefully pick.
This is just from my own personal experience having lived there for a few years. Even though I know some that enjoy living in PR, most statesiders will tell you that the USVI is a much better choice to live in the Carib. If you want some more insight please feel free to send me a PM.