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imply a certain level of accuracy / guidance that I don’t think is at play here for Iran. But maybe I’m wrong

There's nothing definitive on this publicly available. Fateh-313 is assumed to have a low-ish CEP (I've seen claims of anywhere between 5 and 50 meters), which makes it quite capable by anything but western standards. And obviously their missiles are by definition guided.
They've been heavily focused on their missile program for a long time now, especially since entering the nuclear deal. It would be very strange if they didn't make good progress.
Someone explain the logic please: you build a base underground to hide your equipment from us and go ahead to do a grand opening? >

The grand opening doesn't make sense unless it was basically obvious to other militaries that there's a bunker airbase there. Bunkers by themselves honestly are not as impenetrable as they once were and gain a decent amount of protection today based on secrecy rather than actual protection. This is because once using bunkers to store key equipment became commonplace, bunker busters were extensively researched to the point that most nuclear bunkers even aren't able to withstand modern munitions, especially against an enemy that knows where your base is and concentrates on it. In all, this is more for show as a country like the U.S. could either destroy it or simply damage it to the point it no longer can serve as an airbase and only smaller regional powers that have poor militaries would be threatened by it. >

I wouldn't be so sure. With modern advances in concrete technology bunker busters are again having trouble punching through to properly designed bunkers, and Iran is on top of implementing this technology. It's basically the one technological field where Iran is actually on the cutting edge (at least according to publicly available information).
The arms race between concrete tech and bunker busters has been going back and forth for a long time now, and it's not over yet. >

Keep in mind, you don't have to take out every plane in the bunker.
Just the entrance / exit. >

Temporarily disabling whatever function the bunker has is easy enough (crater a runway, destroy any communication links, destroy entrance/exits). It's the permanent destruction of the assets inside that is looking more unlikely. >

The thing about security is, it's the same ideas no matter what you're attempting to secure.
Confidentiality, Integrity, and Availability.
The bunker covers those first two, but limits the last. If that limitation has been mitigated elsewhere, fine. Security is about trade offs. But having a bunch of airplanes grounded because you can't get the blast doors to your bunker open seems like a horrible point of failure in planning. >

Having hardened base is not a bad idea but Iran's fighter is rather outdated to say the least. >

It's an old idea. 80's old. >

Third generation fighters are no match for our fifth gen fighters. And they know this. Those phantoms may as well stay underground and turn the base into a museum. >

They're still usable as plain bombers, launching stand-off munitions from relatively safe airspace. Even a dinosaur like the Su-24 can fulfill such a simple role. After all, Iran isn't interested in contesting its enemies symmetrically, but rather employing large amounts of loitering munitions and ballistic/cruise missiles. In any case you're right that their aging fleet of jets isn't considered a serious threat compared to their ground launched inventory. 
Not like the Su-35 is going to close the gap considerably anyways, as Iranian enemies will be manhandling them using F-35s. >

Typically the words - stand off munitions - imply a certain level of accuracy / guidance that I don’t think is at play here for Iran. But maybe I’m wrong