okay, I understand. I guess I need to educate myself on editing GRUB settings.
thanks for all your help
I'm going to mark the thread as solved and hope that I never have to type "openSUSE Leap 15.1" ever again

Given that 15.2 should be released soon (as it was scheduled originally for a May release), there's a good chance that wish will come true.

Good plan. Without understanding Grub, multiboot can be quite vexing.

Something to think about after getting better settled in is use of volume LABELs. They can be sufficiently unique that none of the worries that lead to use of UUIDs need be any concern. It makes custom.cfg much easier to create, and of little need to maintain. Same goes for /etc/fstab, easy to tell what you see there, and customize should you wish to, for your each of your installations.

Another thought: take a hint from openSUSE. See its symlinks in /boot/? Create them for Debian and Fedora, use symlinks in custom.cfg, and rarely need to touch it thereafter. Debian already has them, except they're in /. The custom.cfg I'm looking at currently is only 3680 bytes for 12 entries, three of which are for starting new (NET) installations, for 6 total OS installations. The grub.cfg created by openSUSE is 30k, approaching 10X its size.

Too many syllables I agree.

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15.1 was scheduled for May 2019, and it was released accordingly. 15.2 is scheduled for May 2020, and the second of its alphas have already hit the mirrors .

Well, in that case, slightly lowered likelihood of the wish coming true.