Oh this is a lot to process, ha. Here's some thoughts around the suggestions.

Alright so about the hefty casting cost problem of dragons in general. Yeah. Now that I've had a chance to play the deck a bit, I'm running into the same problems I had with Atarka. With so much ramp + treasure support (treasures are particularly rampant in my playgroup), Commander in general is so fast these days and this deck had a very difficult time keeping up. I really like the brew I have here, but I need to consistently ramp to have any hope of getting this battlecruiser off the ground. I need to start thinking about fast mana (Ancient Tomb, Mana Crypt etc) because I'm finding that some games don't go beyond 5 turns these days. Dark times for a Timmy like me.

Yeah, maybe Profet93. Thundermaw is decent stat-wise for cost and it's less about the damage portion of it's ability, more about clearing a path for my dragons to fly in thanks to it's tapping ability. But I know what you mean, it does seem kind of underwhelming.

Thanks DareiJuxis. Strionic Resonator is a great card and I had it in the pile of cards I wanted to include, but choices and cuts were getting really tough so I opted for the static ability of Panharmonicon rather than the pay to activate option of Strionic. Also, there's a few more ETB effects beyond Miirym + the 2 you mentioned: Astral Dragon, Ganax, Astral Hunter, Obsidian Charmaw and Thundermaw Hellkite. Still! There's a bunch of attack and death triggers I could take advantage of with Strionic as well so I might try to sub it in to give it a try.

You're right Nintura, all valid points. I'm going to play the deck a bit first to see how it goes, but I've added Zirilan to the maybeboard for now.

Yep, those are solid inclusions DaigarStrasis. Cuts/substitutions are getting tricky at this point, but I'm tempted to sub in Cursed Mirror over a lesser ramp spell like Rampant Growth.

For sure, repeatable tutor + Sneak Attack effects are great. Thinking about it further though, you're paying 5 to bring out Zirilan and unless you have a haste enabler, Zirilan isn't tutoring for another turn, giving the table a chance to respond. If someone spot removes Z, you've essentially wasted a turn.

It could be great if the conditions are perfect; I've got Miirym out + Lightning Greaves on Zirilan giving him haste and protection, allowing Z to tutor right away and netting me another token copy of whatever I bring up. That's still an 8 mana investment. To play devil's advocate again, would it not be better at that point to just run something less setup heavy like Dragonstorm or Tooth and Nail for 1 more mana? Not repeatable, but if either of those resolve it's a big problem for the table.

In terms of mana efficiency (funny to talk about mana efficiency in a big derpy dragon deck but we can try), there's the 1 mana creature tutors like Gamble, Sylvan Tutor or Worldly Tutor. So yeah, there's a few options that exist in RUG.