As I understand it, the "no on-going series" rule is meant to maintain author anonymity before reveals. Mainly, it's meant to ensure each recipient receives a full standalone fic that doesn't depend on having to read other fics for context. Your fest fics are all self-contained and you're not adding them to your series until after reveals, so I think you're in the clear. Though if you're concerned you may want to check with fest mods.

(To me a "series" on AO3 is very general. It can be a single universe, but not always. I have a couple of series where the linking feature is format, e.g. drabbles or poems. I know of series where the fics were/are written for specific challenges.