First of all: this is hardly 'these are plot holes' in a bad way, I just... noticed in thinking about Golbez that there's some curious gaps in his story; things that must have happened given what we already have.
First of all, a general list of things we know about Golbez's post-Flood of Darkness history; this is roughly chronological as far as I can tell, but not really trying to piece together an actual timeline.
- Like everybody else in the Thirteenth, he's been in some way active since the Flood.
- He captured Azdaja sometime soon after she crossed over, which is roughly shortly before the Fourth Umbral Calamity; Azdaja said it was him, and if she'd been around for any prolonged period before, someone would've seen her.
- He recruited the Four Fiends sometime after catching Azdaja.
- He killed the Thirteenth's version of the Watcher, who was confirmably on the moon, when some form of the Zodiark wards were up (they're visible in that flashback).
- He's been working out of a base in the Zodiark Hole since, at the earliest, 6.1.
- The grave of Actual Golbez suggests the moon's basically been an empty location, barren enough to leave an undisturbed grave.
There are enormous gaps in this timeline as far as actual chronology goes, as expected from someone who's basically been on his feet for ten thousand years. That isn't a problem, but it is an admission that he very easily could've been doing a whole lot of stuff we haven't seen. But those last three points are the things that grab me.
First of all: How, when and why did he get to the moon? We actually have no temporal indication of when he fought his Watcher, save that it was before we met ours; we can also tell that while Golbez does control some amount of transit to the moon (given that bunch of voidsent he sicced on us), but again going by Real Golbez's grave he seems to be effectively alone up there. So... when did Golbez make his presumably solo trip? And how, since the moon isn't exactly an easy place to get to? This could've been after Azdaja, which would've given him some level of draconic power, but that's not necessarily a given, nor is it a complete solution, since the actual process of transit is still a big ask even if you've got the energy. And then after all that... why did he bother? Did he know there was something cool on the moon? If so, how? Because he's not exactly an explorer; he wouldn't have gone 'I'm going there because it's there'.
Second, and probably the more important one... If we're the ones that caused the Zodiark Hole to be empty, then Golbez can't have been there for very long at all. Which means, as far as I can tell: either Golbez has an only-recently-vacated domain somewhere, or he killed part of Zodiark before we did. (Realistically, both could be true simultaneously, but I doubt it.) Both of those feel really juicy to dig into; Golbez has very clearly been up to stuff, and we've only seen just enough peeks at it to know there's a story there, and not exactly one I can be confident I'd get right by just guessing.
I'm sure the pre-Flood stuff that we're definitely gonna get about him is interesting, but I really hope we get more information on some of the post-Flood stuff, because I have way more questions about it.