/dev/sda5 is logical partition while traditionally only primary partitions were bootable. I think that today openSUSE is using syslinux MBR for “generic boot code” and syslinux is capable of scanning extended partition for boot flag. You would need to open bug report to get some statement from developers - whether it is an oversight or intentional behavior.
What exact error you get in installer? You should be able to install bootloader on any partition while unchecking “install generic code in MBR” and “set active flag for Boot partition”. Please post exact settings you do and error you get.
If you want a /boot 500 meg is fine need space for 3 kernels. Do you need one or just following old custom??? I believe you can’t use snapper recovery from grub with BTRFS if you have separate boot so today there is a downside.