Try this.
At Ubuntu grub menu, go to grub prompt (grub >) by pressing 'c'
Then type in the following (without 'grub> '), making sure of typo (upper/lower case matters)
grub> insmod part_gpt
grub> insmod fat
grub> search --file --set=root /vmlinuz-linux
grub> search --file --set=xyz /etc/KaOS-release
grub> probe -u $xyz --set=pqr
grub> linux /vmlinuz-linux root=UUID=$pqr rw
grub> initrd /initramfs-linux.img
grub> boot
If you encounter any error, print out the error message with the command.
If you successfully boot into KaOS, let us know and we'll write out an entry and put into the Ubuntu grub menu.
And then, if you still want rEFInd or systemd-boot or both for KaOS, let us know. But this can be optional if you can boot KaOS from your Ubuntu grub menu which will have a permanent KaOS entry without needing to 'update-grub' for any kernel changes in KaOS.