I don't usually have an issue with opening someone up lol. Most of the time, I can condition the opponent to block how I want them to block by using Spin Cycle (B3, 2) so they'll block that last overhead then I'll mix up with Drill Kick. And if I'm not mistaken, B3, 2 has advantage on block.
Other than that, Staff Stance overhead in general will sometimes catch people off guard and you can full combo from that. You can also get crafty with F3 holds and cancels which, depending on position, can also be uses for a follow up combo.
Armored Shroud is a fantastic addition to Pyro though. And it may still be better, but I don't think opening someone up is an issue with Naginata.
But this is just my experience. Still learning so I'm open to suggestions lol.