If you're interested in using open-source tax software for analyzing policy changes, here are a few really cool projects written in Python:

- https://taxcalc.pslmodels.org/

  - webapp here: compute.studio/PSLmodels/Tax-Brain/new/

- https://github.com/nikhilwoodruff/openfisca-uk

  - webapp here: https://uk.policyengine.org/

(disclosure: I'm one of the maintainers of Tax-Calculator and Compute Studio)