LTSpice! It is always useful to be able to simulate RC circuits and basic opamps.
Softwares for designing PCB boards! KiCad is cross-platform and free. However, it has a steeper learning curve. Circuitmaker is a free program that runs on Windows. It is easy to use and uses the Altium engine.
It would be great to have Solidworks or TinkerCAD to create 3D models for enclosures of wearables, and other items that will work with your PCBs.
The machine learning component is also becoming more important. Python is great because it can connect to Raspberry Pi, create websites through Flask and Django, and can also perform machine learning via scikit-learn or pandas.
Enjoy~