Not enough concrete to secure every bolt hole of toilet flange to concrete

Trying to set a toilet flange over this concrete slab. My plumber sleeved the 3" PVC before concrete was poured. Problem is because if that void i cannot get a tapcon into each of the 6 holes...only the bottom 3. What's the best option here? I was thinking to fill the void created by the foam with hydraulic cement, then embed a tapcon while it cures.