We already have Marignon-inspired Ulm in the form of Iron Faith; an Ulm-inspired Marignon would be ok too, I suppose.
1) In the Renaissance the power of the church waned. I think holy-3 priests would be plenty, plus Marignon has plenty of strong priest themes already.
2) In the Renaissance technology advanced. Crossbowmen could be replaced with Musketeers or an even less advanced firearm (Arquebus?
).
3) I would suggest that the advance of technology should also reduce magic. In addition to having weak mages, there should be a mandatory magic-0 or worse.
4) I like the forge bonus on the Renaissance Masters, but they shouldn't have Earth so they are different from Ulm. Also note that with 4 picks which may stack, they are still pretty fair mages and spectacular forgers.
5) I think you have to keep the cavalry around. Cavalry didn't really disappear until World War 1.
6) Given the advances in business during the renaissance, I think having well equipped, high cost units is OK (also works with your "expensive cavalry" idea).
7) National heroes: Da Vinci himself, a spectacular researcher/engineer. Machiavelli, who attracts assassins. One of the various Renaissance artists, who could produce money (is this possible?) I like heroes that do things other than just be mages/warriors.
8) Given the advance of science during this period, Astral would be an appropriate choice (and will also have the effect of keeping the Masters - who could probably use a better name - in the lab).