Failing to Multiboot OpenBSD - stuck at partitions and Linux GRUB

I am trying my hand at learning multibooting, so far my target PC has Antix-Linux ext4 on one partition, FreeBSD UFS on another, ExFat common data on a 3rd, Target PC has been partitioned with GPARTED and GPT partitioning scheme as under: My PC has BIOS not UEFI.

/dev/sda1 - 1 GB - fat32 - flags - boot, esp
 /dev/sda2 - 3 GB - flags - linux-swap
 /dev/sda3 - 15 GB - ext4 - Antix_Linux
 /dev/sda4 - 250 GB - exfat
 /dev/sda5 - 15 GB - freebsd_UFS
 /dev/sda6 - 0.5 GB - openbsd boot (formerly type 'EF' now type '0D')
 /dev/sda7 - rest space 15 GB -openbsd 'A6' type UFS

1] OpenBSD with Ventoy - installed from the install77.iso file, downloaded filesets from http since it did not read from cd0 or disk, got stuck at installboot. Then it was showing error EFI device busy, and since I don't have EFI, I changed the /dev/sda6from type EF to 0D(boot bios) and installboot successfully completed. Then I rebooted, and trying to add a menuentry in 40_custom grub file in Antix-Linux, whose grub boots the system, but update-grub is showing warning discarded incorrectly nested partition hostdisk/dev/sda,gpt7,bsd(1-14) May you please extend your help in these cases. Regards.