I have an idea though. Create a new tag. Call it release, include the Flatpaks in this tag, use the tag as the default in the installer, then post install you get rebased to latest without the Flatpaks built into the image. They should still be a part of the system
Kinoite doesn't ship with any flatpaks out of the box. When you rebase from Silverblue to Kinoite, they do not get installed because there aren't any and you get left with the SB flatpaks, but likewise, when you rebase from Kinoite to Silverblue no flatpaks are installed because they are only installed during installation itself
i put together https://blincus.dev in maybe 30 minutes. Still missing lots of words, but getting all that nice plumbing done in half an hour is a nice bonus.