Would installing an OS on an external ssd and booting into that to run pirated software while blocking access to other drives in your system or physically unplugging them be one way?
Or are there better ways to isolate the software you run and use as much as possible?
Depends on your threat model.
A air gap system is the gold standard.
A virtual machine is a reasonable middle ground, and of course you cut the network access.
Qubes lets you do both but it sacrifices some performance.
A word of caution about dual booting systems: if something is running on the computer, it in theory has full access to everything attached to that computer, including unmounted drives, encrypted drives, even BIOS. There are Trojans that install themselves in the boot partition, and it’s possible an infected operating system could infect the non-infected operating system next time you boot.