You failed to tell us what went wrong with your attempts.
Well, okay, I see the title line which is probably an indication of trouble.
You appear to have a UEFI system. And it looks as if Windows is installed for UEFI booting.
If you want this to work, then you will need to boot the install media in UEFI mode.
I don’t know the details of your computer. Apparently some systems are confusing with respect to boot mode.
My suggestions:
(1) Disable secure-boot in your BIOS. That’s because there are bugs in the opensuse installer. After your system us up and running and you fully updated it, then you can re-enable secure-boot.
(2) Disable legacy boot options in your BIOS. That is sometimes called CSM (compatibility support module). If you can disable that, it should ensure that you boot the installed media in UEFI mode.
(3) I’m guessing about how you wanted to partition. So recognize that I might have guessed wrongly.
In the partitioning section, select the option “Create partitioning”
On the next screen, select “Custom partitioning” (and ignore that warning that this is for experts).
That should give you a screen showing your existing partitions. On each partition, you can change how it will be used. To do that, right-click on the partition and select “Edit”.
Here’s what I think you want:
“/dev/sda5”: You will want to mount that as “/” (your root partition). And you will probably want to set it to be formatted. It’s probably okay to keep the current subvolume structure. I have not reinstalled into an existing “btrfs” file system, so I’m not quite sure what the prompts will be.
“/dev/sda2”: You will need this to be mounted at “/boot/efi”. DO NOT reformat. Just use the exising FAT formatting.
“/dev/sdb3”: You probably want this to be mounted at “/home”. It should not need to be formatted, unless you have some special reason for wanting to format it.
“/sda4” and “/sdb4”: You possibly want to mount these as “/windows/C” and “/windows/D” (or similar), so that you can access Windows data in your opensuse system. You can make appropriate decisions about that.
With those choices, your install should work. Your problem with bootloader install was either because you did not boot the install media in UEFI mode or because you failed to mount “/dev/sda2” at “/boot/efi”. It’s a bit hard to guess which of those went wrong.
I hope this helps.