Update EFI/grub to dualboot two distribs from single drive with btrfs

I'm trying to create dualboot system with openSuse (15.4) and Ubuntu (20.04).

I had Suse installed on a drive /dev/sda with following structure:

/dev/sda
|- /dev/sda1
  |- /boot/efi              (vfat)
|- /dev/sda2
  |- /                      (btrfs subvol @/.snapshots/1/snapshot)
  |- /boot/grub2/x86_64-efi (btrfs subvol @/boot/grub2/x86_64-efi)
  |- /boot/grub2/i386-pc    (btrfs subvol @/boot/grub2/i386-pc)
  |- /home                  (btrfs subvol @/home)
  ...

The idea was to create separate subvol for ubuntu / and share all other subvols like /home

I've created new subvol @/ubuntu-root mounted it to /mnt/ubuntu-root:

|- /dev/sda2
  ...
  |- /mnt/ubuntu-root       (btrfs subvol @/ubuntu-root)

Afer that I debootstrap'd ubutntu into it.

I can successfully chroot into freshly installed ubuntu, but I can't come up with the way to properly update EFI loader and grub.

I'm really new to all grub stuff and all guides are really messy in terms of what comes first and what should be manually updated and what should be generated by what. ALso, and none of them seem to mention EFI.

update-grub mentioned in answers to this question does not exist on Suse, os-prober does not seem to detect ubuntu from Suse thus grub2-mkconfig does not change anything.

Is there some place where All this stuff is concisely explained, or maybe there's a guide that could help me but I can't somehow find on google?

UPD#1: I've installed proper kernel (5.4) and done update-initramfs -c -k 5.4.0-26-generic from chroot. Now, grub-mkconfig from ubuntu chroot generates somewhat proper config for ubuntu with proper menuentries and load targets:

...
menuentry 'Ubuntu' ...
...
linux   /@/ubuntu-root/boot/vmlinuz-5.4.0-26-generic

However, it does not see existing openSuse (obviously, I guess)

As far as I understand there's following order of things happening:

1. BIOS loads EFI from /dev/sda1 which is /boot/efi/EFI/boot/bootx64.efi
2. /boot/efi/EFI/boot/bootx64.efi has hardcoded path to /boot/efi/EFI/opensuse/grub.cfg which it tries to load
3. /boot/efi/EFI/opensuse/grub.cfg has follwoing contents

set btrfs_relative_path="yes"
search --fs-uuid --set=root {UUID}
set prefix=(${root})/boot/grub2
source "${prefix}/grub.cfg"

which I ASSUME mounts default btrfs subvolume from FS on /dev/sda2 and loads next grub config from that subvolume /boot/grub2/grub.cfg that resides on opensuse root subvolume.

I can manually alter that config and insert ubuntu menuentries but those would be on subvolume that is not mounted, since default one is mounted outside that config and before any OS choices

Can I replace grub.cfg on /boot/efi/EFI with one from OS root subvolume? Or there are some tools that should do that for me?!