Follow this recommendations or you never leave the boot manager hell:
Configure your system to boot in UEFI Secure boot mode.
https://docs.oracle.com/en/operating-systems/oracle-linux/secure-boot/sboot-OverviewofSecureBoot.html#sb-overview
Follow:
Each operating system should have one directory on the EFI partition. One directory for TW, one for LEAP (and one for MS Windows). You will find in this directory the EFI file which contains code from the boot manager for this operating system.
/boot/efi/EFI/opensuse/grub.efi → LEAP, UEFI, but without secure boot
/boot/efi/EFI/opensuse/shim.efi → LEAP with UEFI secure boot
/boot/efi/EFI/tumbleweed/grub.efi → TW, UEFI, but without secure boot
/boot/efi/EFI/tumbleweed/shim.efi → TW with UEFI secure boot
/boot/efi/EFI/Microsoft/bootx64.efi → MS Windows 10/11 with UEFI secure boot
/boot/efi/EFI/Microsoft/bootmgr.efi`
Each operating system should have his own boot entry in BIOS (UEFI firmware) which loads directly the correct EFI file in the correct EFI directory. Fix and check boot entries in BIOS with efibootmgr:
https://en.opensuse.org/openSUSE:UEFI
Use BIOS boot menu to start the operating system of your choice.