After reading all this - I think I'm going to swap my 721's for Flow Ex's, benefits being they're 100g lighter (only 20g more than Flow's), tubeless (obviously) and have the same ERD as the 721's - so a straight rim swap without needing new spokes is possible.
I'm guessing they won't be as bombproof as the 721's - but I don't ride them hard enough to justify needing all that strength/weight.