Congratulations on a beautiful cake! Beginner’s luck, indeed!
Air holes in frosting are fine for a buttercream. If you want to minimize the holes, make it a day ahead, bring to room temp, and rebeat with a hand whisk, for about 1 minute. The buttercream will be a little denser and smoother, with fewer air holes. But it will also seem less fluffy when you eat it. I like this “re-whisked” texture for its smoothness/silkiness, but some prefer the fluffier version.
Was your cake at a cooler temp when you ate it? Like butter, buttercream is firmer at cooler temps and softer at warmer temps.