Physically you’d have N+2 partitions for N systems: one for swap, another for storage, and N for system files. Then inside each system you’d have simply to mount the swap, /, and /home/user/storage.
I recommended OP to turn EFI off, it sounds pointless in his situation. Regarding /boot: it boils down to installing grub in one of those distros, and letting it manage the boot.