@computersavvy - there are lot of really useful observations and info there thank you very much!
I created another topic as that exercise is worthwhile stand-alone, imho, but I have also been playing that grub.cfg file you mentioned, and it seems the UUID there is used to populate the kernel commands in the top grub-menu entry, but there must be more to it than that as just editing the UUID and getting a grub entry pointing to the old partition is not enough to get booted.
One other thing I noticed is that when I run grub2-mkconfig from “emergency mode” it says something like nvme0n1p1 allocation flag deprecated which is not something I’ve encountered in working installations of fedora.
I also tried chrooting as described in the article about reinstalling grub2 and also systemd-nspawn -D /mountpoint-to-old-fed but then grub-probe (called indirectly) complains about not being able to access /dev stuff and I could not find any way to mount -o bind them (also need the right /boot/efi to be mounted otherwise files are just put in the directory rather than the mount target nvme0n1p1), and even if I did, I am not convinced it would work: from chroot or systemd-nspawn I was hoping to be able to reinstall the kernel, reinstall grub2, grubs2-mkcongi, a few dracuts here and there and … that’s pretty much the extent of fancy words I’ve picked up so far, but it is clearly not enough.