Mathematica is certainly as powerful as a TI-89, but not OSS.
Right, though you can get a free license with raspberry pi, I think? https://www.wolfram.com/raspberry-pi/
I'm sure there are restrictions, but likely not a concern for hobby interests?
Many symbolic options exist in lisps. https://stackoverflow.com/questions/10355112/why-is-lisp-so-... is a good answer that goes over some of the reason for that.