I have not looked at this project for since 2022, but noticed it today and wondered if there was an assembler, because the firmware would need editing to use a different communications device. I expected it would run on a TI99 computer because there would be little demand for tools for this brief-lived processor.

But there are!

http://john.ccac.rwth-aachen.de:8000/as/ cross-assembles to many obsolete processors, such as the 9995, Z8000, PDP-11, etc. I shall definitely look at it for my Z8000 project.

https://endlos99.github.io/xdt99/ is a whole suite of tools for the TMS99xx family of processors. Assembler, disassembler, etc. All written in Python, so it will run on any platform that has Python 3.8.

Etched Pixels has created a C compiler for it:
https://github.com/EtchedPixels/pcc-tms9995-support

I shall ask Stuart Conner for the source code for the BASIC and FORTH.