HN user
Wow, that's a quite detailed response, thank you.
I need to check out DMDE disk editor, because the last disk editor in DOS that I used was an old copy of PartitionMagic with Win9x-like interface.
Also, I remembered that old boards without UEFI rarely came with a flash utility inside BIOS, so you need to boot to DOS and use AMIFLASH or some other utility to update it.
Did you had luck with newer motherboards and DOS? I've seen IDE compatibility mode for SATA removed and DOS can't access disks even for diagnostics, like Victoria HDD.
And what use cases do you have in DOS that can't be done in Windows/Linux Live CD?
3.5 mm to 3.5 mm cable with decent length would be nice to have if you need to hook up a laptop/phone to speakers. A spare PC power cord and a figure 8 cable would also be nice. I've seen many cheap cables that caused sparks because of a bad plug. One millimeter off and computer shuts off.
One Philips PH02 screwdriver if you ever need to clean a PC or change SSD, memory, etc.
For the software: for Windows side you can use Hiren's BootCD (you can use USB drive despite it's name). It doesn't contain pirated software as Sergei Strelec WinPE.
As for Linux - well, you can use Ubuntu's Live USB/DVD to be able to get a running system in minutes to recover files or do other stuff.