Partitioning in the Debian installer being half-broken is something nobody talks about but IME still a thing.
What do is step through the installer to the point where you're at, ctrl+F* to get a shell, set it up manually using fdisk/mdadm/lvm/cryptsetup/mkfs, and then back again to rescan and just assign the mounts and filesystems
I think I still have a half-written guide for just this in drafts somewhere actually. If you get stuck you can DM and maybe I dig something up