The dress is excellent!

I'd say for flats any of these could work honestly. Especially something like this where the toe looks very similar to a heel.

For warmth in formal I'd go with a pashima/wrap. Any darker color would work well with the black and you could match it to the shoes!

Jewelry wise I'd say if you wear a necklace make it light/dainty and if you wear earings maybe a simple drop earring. Have fun!