I'll fully admit I'm not as well versed in web programming as it's not my specialty, but aren't serverless functions (e.g. AWS Lambda) essentially the same concept?
I understand that they can solve the problem of horizontal scale as they are spawning a container rather than just a process, but surely if you started with CGI scripts it would be easier to move if you needed to at a later date.
Frink! https://frinklang.org/ sounds exactly what you want. It has built in bignum support and mathematical functions. It also does units and their conversation natively.
It runs on top of the JVM so there's an Android port also. The Android app appears as two icons. One as a simple calculator and the other as a simple scripting editor for more complex requirements.
I understand that they can solve the problem of horizontal scale as they are spawning a container rather than just a process, but surely if you started with CGI scripts it would be easier to move if you needed to at a later date.