Maxima[1] is also a very cool symbolic math system. They should migrate to GitHub or GitLab though for a better visibility and lower barrier for external contributions.
When I temporarily gave up trying to apply myself towards my math coursework during high school, I used Maxima to breeze through an online Algebra 2 class. Not a great story, but I can attest that it works.
Sagemath includes a lot more math, esp. number theoretic stuff, and is a quite large. (~1.6G compressed) Sympy is _much_ smaller and integrates more easily into python code that isn't solely mathematical.