I searched:
https://html.duckduckgo.com/html?q=HCS08not even realizing it was for the old 68HC08. That's a processor from the '80-ies or so I think?
The first that popped up was an IDE from cosmic. I have no idea about the quality of their tools.
But for that old stuff it seems like a complete revision of the project is in order, especially for > 10k production volumes. Lots of things have progressed in uC land and such an old uC is not quick and may be replaced by a 40ct part. Apparently you use C compiler, and C-code is relatively easy to port to any microcontroller, especially if it's well written and not too heavy on I/O or dependent on special features from a particular microcontroller.
With those old parts the availability of the hardware itself may also become a problem.
I did not read the whole thread, but to me it looks like it's getting time to have a broad view look at the whole project and set out a strategy for it that will work for the next 10+ years.
I myself have a preference to keep things simple.
Just give me a decent C compiler and some header files and startup code.
I really do not like makefiles, but once you've got a project working with GCC and make, you usually have a setup that is relatively stable and reasonably future proof.