So, my iPhone 15 arrived about 90 minutes ago. Regarding the software update, I would do it differently if starting from scratch.
I responded to the initial prompt to migrate from my iPhone 14. I was hoping it would then notice that the software update was needed. However, it didn’t do that. It asked if I wanted to transfer the old phone number now. I refused, and after some delay, the new phone restarted. Now, it noticed that the Software update was needed and requested a connection to WiFi; once it found nearby WiFi, I selected my network; the iPhone connected without asking for the password, updated, and restarted. At this point, it again asked how I wanted to proceed, and I again went through the procedure to mate with my old phone. After responding to various prompts, including enabling my credit cards for Apple Pay, it transferred data for about 45 minutes and restarted. As I write this, the new phone is partially functional, as the actual app updates need to come from the cloud.
If I were to do this again, I would respond initially to set up the phone as new. I would run through any prompts with dummy information until I reached the software update required notice. I think I would need to supply the password for the WiFi network, but the whole process whould run faster as I would skip the initial handshaking between the two phones.