... "the 8051 is great at doing the things it was designed to do, and very poor at doing things it was not designed to do."

"Being programmed without a host computer" is one of the things the '51 was not designed to do (along with other items like "floating point maths", "dynamic memory allocation", "self-modifying code", "using function pointers" and "running an OS that doesn't qualify as 'simple'").

While it is certainly possible to do all of these things on a '51 (maybe not on every chip, but you'll find a derivative that can), you will usually have a much easier time if you don't work against the hardware architecture and your tools, and pick a different chip if you intend to do any of these things for anything else than an academic exercise.

And welcome to the forum. :)