Hi, Pascal:
Thank you for following through here. Well, OK. But this is a very puzzling choice to make, as Bookworm live images could set up the APT mirrors, as noted in the same e-mail thread by someone else. Why not with Trixie? Why assume an offline installation? Why the change? Why break things if they worked in the first place? It all just seems so counterintuitive. For now, I will use the Calamares installer going forward. That does the trick. But if this doesn't change; the decision that was inexplicably made to not offer network mirrors, etc., during installation with the "expert" mode (which I now understand happens when booting directly from the USB and not through the "live install" offered on the desktop in live images) really should be articulated clearly on the Debian website. But I really would respectfully suggest (as an end user) that things go back to the way they were with Bookworm live images (for Trixie). I can only imagine how many confused people there are out there dealing with the same thing.
Thank you again.
Max