HP48 and FORTH
On these days I stumbled upon Forth, a 1970 programming language which is quite similar to HP48 internal programming language.
Like the HP48 language, Forth uses RPN, it is stack based and have some success for being tiny and easy to implement.
For a 6502 Forth compiler you can refer to https://github.com/piotr-wiszowaty/foco65
For a list of Forth rimplementation refer to this link: https://theultimatebenchmark.org/systems.html