And for all the complexity of the tax code, it often pales in comparison to the version of the tax code we subject low-income families to: rules for means-tested benefits.
France has also developed the OpenFisca framework [1] for tax and benefit rules as code, and its OpenFisca France [2] model is widely used (I think significantly more than mlang). We've extended it to the UK [3] and the US [4].
Fantastic job making taxsim more accessible! We've built an open-source version of taxsim called OpenFisca US [1] which we expose in our (also open-source) web app, PolicyEngine US [2]. We haven't yet implemented all of taxsim—we only have the most recent couple of years, and not all states—but we have some advantages like implementing means-tested benefits and being able to simulate custom tax and benefit reforms, both on individuals and the population (e.g. the budgetary and poverty impacts).
My team at PolicyEngine [1] is also now further reimplementing Tax-Calculator in the Python-based OpenFisca framework [2]. OpenFisca US [3] includes all tax logic in Tax-Calculator, plus many means-tested benefit programs like SNAP, and some state tax logic (currently only Massachusetts is complete, though we'll finish the country in the next 12-18 months). You can try it in our PolicyEngine US web app [4].
(OpenFisca US is part of the Policy Simulation Library, and it's developed by a number of former Tax-Calculator developers, myself included.)
SmartAsset is only a rough approximation based on standard deductions/exemptions and the rate structure. It excludes tax credits and other provisions that make taxsim much more accurate, especially for low-income households.
Our current static approach follows the bulk of public policy analysis, but we do plan to add dynamic responses. For example, https://compute.studio/PSLmodels/Tax-Brain which supports "partial equilibrium" which adjusts earnings based on user-provided labor supply elasticities. https://compute.studio/PSLmodels/OG-USA performs general equilibrium analysis, which also considers feedback loops labor and capital over decades, though it takes several hours to solve. (We've contributed to both of these open-source projects.)
These labor supply elasticities and other parameter values are debated (especially when it comes to deficits and corporate taxes), so we're starting with the less controversial results while also describing changes to labor incentives through the marginal tax rate charts.
PolicyEngine lets anyone simulate reforms to the UK tax and benefit system, and see the impact on society and their own household. We're working on building it for the US as well. Here's a two minute demo video: https://youtu.be/P7ELqxEHRhE