IMO it is very fair to consider the CCC of a trailer in the equation - I've always pushed back against the generalized GVWR advice for that very reason. With the wide variance of ccc limits on these trailers, you have to be aware of it. A 4k trailer with a GVWR of 6k will be no different than a 4k trailer with a GVWR of 7k. Both start at the same weight and will be loaded the same - to consider one as ok and the other as too much is silly. Furthermore, you could find a 5k trailer with only 1k in ccc, which would also be 'ok' by the GVWR rule, and I wouldn't touch a trailer like that with a 10 foot pole!
That said, there are two things I'd add to your consideration...
First, the one piece I see missing from your calculations is the hitch weight. Your WDH is going to weigh another 100 pounds. If you are down to 720 after your family and gear, your real tongue weight limit is close to 620.
Second, counting on 12% is a bit risky IMHO. While 12% is certainly within the range of acceptable weights, every trailer is a bit different. If your loaded trailer comes in more naturally at 14% when loaded, how are you going to reduce the ratio? Shorter trailers have less opportunity to shift the load behind the axles to accomplish this, so you could find yourself overloaded with a heavier tongue and no way to adjust it.