GRUB2-BLS does not work with dual booting so that is not an option in your case.
For a 15.6 system using GRUB2-EFI I would keep the -EFI bootloader for the upgraded system. If you really don’t like it you can change that later, but it isn’t a trivial procedure (see for instance systemd-boot installation).
Choosing GRUB2-EFI is “safe”; I cannot tell whether or not there will be “problems later” since I never upgraded Leap 15.6 to Slowroll but upgrade of 15.6 to Tumbleweed is a routine openQA test, so that might depend on how “standard” your Leap 15.6 install is.
For instance, I would disable all non-default repositories and do a full update (for instance by zypper dup --allow-vendor-change) before the upgrade.