thank you for your welcome and replies.

My kids are toddlers and I do not want to have stops: it is too annoying to switch international to local airport area (especially Shengen area in Amsterdam) or pass custom when we enter in the US.

We plan to do the 2 trips / year for several years.

It seems we're not enough flexible on the flight and period to get the best prices. And we don't travel enough to get nice miles rewards.
I read that it is quite difficult to use your Delta skymiles during summer/Christmas period.

It's maybe better to stick with my costco amex and the ~2% cash back.