Now days, it's not often that I need to do heavy mathematical work, but I turn to Sage https://www.sagemath.org/ when I do.

Sage has a notebook interface, is built on Python, and incorporates many free software math packages, like Maxima, into one system.

I used Sage heavily when working in string theory a long time ago. It's very powerful. But for symbolic math, if possible, I would still use mathematica.