Congratulations on the successful result of your year long quest! To me, the crumb looks nice and open. Well done on your first try! Like you, I have stumbled onto some successful strategies by accident while baking. Even some of my greatest failures have given me renewed resolve leading to even greater successes. (One year, the tops of all 3 of my panettone fell off within minutes of inverting them! The following year is when I started having significant successes with this recipe.)
Regarding “spiking” the second dough with additional starter, I have not seen any of the many recipes I have looked at add more starter after the first dough. While it will shorten your final proof time, it may subtly alter the flavor profile of the second dough increasing the acidic “twang”, characteristic to conventional sourdough. I think the citrus content in this recipe is fairly forgiving of some extra acidity, since I think it can punch up the citrus notes. Of course, I think one of the prime reasons for all the attention to 4 hr refreshments leading up to mixing the first dough is to reduce the overall acidity level and perhaps to encourage a milder combination of lactic and acetic acids than you would find in a more mature starter. Last year I made a batch of Matt Tinder’s chocolate panettone and I was a bit more lax on my 4 hr refreshments. When we took the first bite of the panettone, we noticed that it had a bit of that sourdough twang, since there was no citrus to balance it out. Though my wife was a bit disappointed, my in-laws really liked it. So ultimately, if it tastes good to you, then that is what matters most!
Regarding laminating the dough, again I have never seen reference to using that for panettone. In my opinion, the very long leavening times and the pretty spectacular change in volume from when you first load the dough into the molds to when it comes out of the oven usually results in a light, open crumb even without laminating. That said, when I am measuring out the dough, I do some brief coil folding to get the dough back into a ball shape before the “pirlatura” action. You can see what I mean in the longer of the 2 pirlatura videos in the original blog post.
Anyway, congratulations again making your own panettone. I bet you’ll never buy one from the store again!