Slightly bigger, 50% in both directions. I thought I picked a good size, would have liked bigger. When I renovated and redid it at my second pond, going bigger was too much work, so I'm this there.
Smaller PVC. I went with 3" under the pond to the bottom drain. For my small pond, 2" was plenty. I could have done 2x 2" to avoid vacuum possibilities at the same price and with less aggravation. My skimmer has a 1.5" outlet, I have a reducer so it flows into a 2" pipe, even that was silly, 1.5" was plenty for what I was doing.
More plants early on. Fish are fun, but I love what I found for plants, could have used more.
More bog. I love what I did, I would have enjoyed 67% more bog. I used 3 planters, would do 5 if doing again.
In general I was much smarter for v2, but overthought a few things and was slightly limited by v1 decisions.