[gnramires]

Note: I had this response typed up but didn't submit...

One related app that I absolutely love is Qalculate![1] (yes, it has a built-in exclamation for default enthusiasm :) )

It can do cool stuff like converting N (newtons) to kg.m/s^2 when you specify units as ?kg. It also converts units like 1kW x 1year = 31.55... GJ

It's fantastic for engineering and specially back-of-envelope calculations. This notepad aspect does seem useful though. One alternative is to use Jupyter notebooks, sometimes I work problems with Sage[2]+Jupyter. Sage is extremely powerful (you can do calculus, linear algebra, and more) but doesn't support units (that I know of), it's more geared toward advanced maths.

[1] https://qalculate.github.io/

[2] https://www.sagemath.org/ It's a bit on the heavy side although it's definitely worth it if you're doing a lot of math. I think the flatpak is preferred due to its significant size.