Available ColdFire ICs:
https://www.nxp.com/products/product-selector:PRODUCT-SELECT...
Differences between ColdFire and 68k:
There really aren't. You could consider Arm (maybe with the simplified but 16-bit "thumb" instruction set), or MIPS, or RISC-V. Between all of those, it's rare to find bare CPUs; usually you get them in a system-on-a-chip, with things like DDR memory and other sophisticated peripherals built-in. You need complicated drivers and a bootloader, maybe even with signed payloads. Nothing as architecturally simple as wiring the address and data lines of a CPU into the address and data lines of some RAM or 7800 logic directly wired into a keyboard.
In principle those devices could exist, but as a hobbyist you can only really get your hands on hardware with that kind of conceptual simplicity by either programming it into an FPGA, or by using hardware that dates back to when that approach to computer design was cutting-edge.
edit: you could maybe consider some kinds of microcontrollers? The Parallax Propeller comes to mind.
More seriously, since the time of 8-bit CPUs, hardware has essentially split into deeply embedded system on chip designs (often ARM or RiscV cores) or monster multi-core 64 bit CPUs. The intervening 16 and 32 bit processors have largely been consigned to history (genuinely a shame - but that's the reason there are so few kits based on them). Ironically, it's the old Z80 and 6502 that are still being manufactured today in volume and at low cost as a convenient building block for simple "computer" devices.
So, no - this sort of kit does not teach you how your M3 Mac works - but it does provide you with a system that you can completely understand and control, that you can build your own peripherals for, that you can write a complete operating system for, or games and programs, that you can have the satisfaction of building yet still has the basic architecture of any modern computer. And for the sort of task you're likely to undertake, it's surprisingly capable.
Alternatively, if you want hardware with a simple programming model that abstracts the difficult stuff away, get an Arduino. That's really the choice you're making - either 'close to the metal' coding on older CPUs, or the modern approach of abstracting everything into software. Both are equally valid.
(Disclosure: I’m the designer and lead developer on that project).
That said, we _do_ have a slightly-related 6502 too https://github.com/rosco-6502/rosco_6502