I have a similar situation. I too have a Windows VM on a late model PC running Windows 10 64 bit and have some old-old 16 bit software that will not run on a 64 bit OS. It won't work with DOS emulators either, I tried a few of them. In my case the guest VM is Windows 7 32 bit, but that doesn't matter.

I use AUTOLOGON by Sysinternals to automatically log the Windows 10 host PC in. I put a shortcut to the Windows 7 VM in the startup group and have it start full screen. So besides a 20 second flash of Windows 10 when the PC first starts all the user sees is his Windows 7 OS with the software he needs.

Virtualbox presents the console to the VM as an app, rather than being forced to strt the HyperV app and open a console - I could never find a way to automate that.