You're overcomplicating things.
If you have an existing Win10 install, just format the new SSD for whatever distro, install, and set the boot target for the new SSD drive.
Then run Linux for a bit, make sure everything works, and make an image of the already existing Win10 partition to run in a VM.
Much simpler.
Also: almost all tax software runs in a browser now. If not then it will run under Wine or Proton. I think you have less to worry about than you think.