@mrmazda may have some tips for you…
Dunno how much of a guide this is worth:
| 15.6 | 16.0 | 16.1 | TW 64bit
installation count | 34 | 10 | 7 | 38
multiboot | 34 | 10 | 7 | 37
Windows 10 also | 2 | 0 | 0 | 2
install type zypper dup | - | 8 | 6 | -
inst type migration tool | NA | 1 | 0 | 0
install type Agama | - | 1 | 1 | 0
boot Grub EFI | - | 7 | 6 | 7
boot Grub2 | - | 0 | 0 | 2
boot Grub Legacy | - | 3 | 1 | 29
That’s 29 multiboot 64bit PCs, each using only one (openSUSE) bootloader for an average >12 GNU/Linux operating systems per.
I found the migration tool and Agama more frustrating than plain old-fashioned zypper dup using the same manual repo management system I’ve been using since before Leap and TW were born. However, I use a minimal amount of non-default repos, mainly: KDE3 or TDE (no Gnome, almost no Plasma, no XFCE); Packman for AV necessaries; File Commander (1 orthodox file manager); home:ecsos (mainly for latest MC orthodox file manager release).
I should think anyone who could manage to have so much special software installed and working should have nominal trouble determining how to upgrade the old-fashioned way if the migration tool seems too, or turns out to be too, scary. That said, why not add another SDD and use Agama on it, and take as much time as needed to get your customizations installed to it? Or, add another SSD, clone from 15.6 to it with rsync as many times as it takes to get an upgrade via migrator or zypper that works well enough?
@mrmazda : Thank your for the detailed information! 
Once again: Maybe the migration tool Dual boot with a legacy boot windows? - #16 by user42 (staying at AppArmor and re-activating boot-probe) would be the option with the smallest risk for me?
Any experiences about that for my constellation?
Agree, assuming quality backup and/or using a clone on an additional storage device facilitating a do-over if necessary — least disruption potential to a satisfactory state.
Nothing personal that I can think of.
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