It sounds like you could use Qubes OS, Whonix, Tails, or some combination thereof.
Why not install WINE or even use Steam Proton?
Why can't you resize the partition?
Resize your Windows partition and encrypt your linux partition with luks or something like that. Instead of default windows bootloader get grub ot refi
shrink your windows partition and leave some space for linux. if it doesnt let you, it might be too fragmented