not ONE Linux installer (ubuntu,fedora,alpine, popos) support customized layout with luks/lvm on existing partitioned disk. it's the whole disk or nothing.
I spent an unreasonable amount of time circumventing this limitation especially as i didn't want to reinstall and or bootstrap as i'm abroad. I had to do it anyway. Oh and GRUB is a piece of crap (what did they smoke???), how come every distro we didn't move yet full UKI?
I made my life difficult tbh, but i' m up and running.