The transfer from our 12 minis (on 16.6.1) to the new 15s indeed went without a hitch. 
It’s a bit odd that you first select to transfer from a previous iPhone (or iCloud backup) and then the “Hello screen” tells you there are updates available. I would expect the other way around. Anyway, at the same time we were asked if we wanted to migrate our SIM to the new eSIM and that went on in the background.
We had those iOS updates installed (no way to tell if it went straight to 17.0.2 at that point) and then again (why again?) had to select to migrate from a previous iPhone. That ended up going just fine (took about 1/2 an hour though) and once that was done it still needed to download all the apps from the App Store. But again ~20 min later and all was good. At that point I was finally able to verify that indeed we were on the most recent 17.0.2 and that our phones had hooked up to the T-Mobile network (sure enough, by now the old minis were showing only SOS, no more T-Mobile connection). I also received about four emails and texts form T-Mobile telling me we had been successfully ported to the new eSIM.
Most of our settings came over just fine, the exception being Threema (by design) and our banking apps. I wasn’t aware that despite all the security and FaceTime and migrating from one iPhone straight to the next, you still loose history on your Wallet cards, both debit, credit, and transit pass. 
They all work (already verified at stores and using BART), but they show no previous history. That one banking app (credit card) is still refusing to accept FaceTime auth I’m chalking up to iOS 17 being too new or rather, the app not having been updated yet for proper support. I seem to remember vaguely we went through the same baloney last time when iOS 16 came out.
All good so far. Although these updates and the transfer takes fairly long, the fact that it in the end all works without hickups is great! That the whole SIM to eSIM transfer went without a hitch as you’d want it to in your best case scenario and that I didn’t even have to spend one minute on the phone with T-Mobile or in one of their shady stores is the icing on the cake. Well done, Apple. Thank you. 