So my experience was fine, except Japan doesnt do the visa stickers anymore - so I had to ask for one, but again that was because of the excitement of having it in the passport itself to "christen it."

My most recent trip back to Japan, which was in February 2024, I used the US passport this time. I now generally only use the EU one when traveling to Europe as it provides visa-free access and a simpler electronic gate experience. Otherwise, I only use it outside of the EU where it saves me money on Visas.

But you'll be fine if you want to use it as a spercial occasion, in short do the following:
- Check in with the airline using the EU passport
- Go through US security with the US passport for simplicity
- On arrival in Japan, enter with the Italian passport
- On departure out of Japan, check in with the airline on the US passport
- Go through Japanese exit immigration with the EU passport (since you used it to enter)
- Go through US customs with the US passport

Have fun!