Are there any computers on the market that try to replicate the simplicity of earlier home computers? Perhaps there's an SBC that would meet my hardware requirements, and there's a separate, simple OS I could flash?
Neither of the 8080 and Z80 chips were considered as 'simple' for that era. And they were so horribly expensive (say about $800 bucks in today's money) that we were paranoid about zapping them with static electricity.
There were several projects using only standard TTL chips and making a usable computer, but they quickly were displaced by the 'CPU on a chip' when the 8080, Z80, 6800, 6502, 2650 and a bunch of other chips all came out. Eventually the wide selection was whittled down to pretty much just the 6502 (Apple) and the Z80 (The main bunch).
I suppose a good question is: What do you want to do with it?
Do you want to learn programming principles?
Do you want to learn programming a particular CPU?
Do you want to learn programming in machine language (assembly language) or a higher-level language (C, Java, Rust, Python)?
Do you have an embedded computer/hardware combo in mind?
And many other questions like those.
And of course, most of those 40-year-old CPUs are still available to play with, anyway. :)
https://www.youtube.com/watch?v=3KAjZbjPqnA&feature=youtu.be
The earlier chip had that retro feel in the ways you describe, but it was just a bit small for computing.
The current one is pretty nice!
It is an 8 core multiprocessor. That may seem complex, but the way the cores are implemented is clever and generally useful without an OS though some are in development.
Each 250+ Mhz core has 2K private RAM, locally addressable by the core, and another 2K LUT lookup table RAM that can also hold code.
The system has 512Kb shared RAM that all cores have access to and that they can run code out of as well as manipulate data.
Here's the cool stuff:
Each core has a data streamer capable of many signals, display from old school NTSC through VGA AND HDMI. Audio, many other streamer modes and uses are available. One per core! It has a color transform engine meaning one can drive a TV with one, VGA with another, HDMI with another and have the colors all make sense from single or multiple display buffers.
This is all software driven and simple, meaning displays can be any mix of tiles, sprites, bitmaps, whatever.
All 64 pins have a smart pin state machine for measuring signals, other pins, and each pin has 8 bit ADC, DAC capability as well as simple digital mode signaling.
It can boot from a program on SD card, over serial, from eeprom.
Currently, there is a dev environment able to mix and match, SPIN, BASIC and C.
Again, each core runs independently. One can do crazy stuff like run an oscillosope on one core to display the signal output of another one, or an incoming signal from off chip.
Or, have one core emulate a CPU, while the others do hardware, video, sound chips.
In my view, it is a nice, robust system on chip. Definitely one person understandable, and if one wants an OS, write or use one of the ones in progress.
Or, just write simple programs and load up drivers onto the other cores to display, work with USB, serial, drive motors, whatever.
There is a Forth self hosted environment and others are being made.
I have enjoyed these devices for a while now and they are one of the closest things to older 8 bit systems around.
These are 32 bit systems and there are a lot of features I did not mention.
It may be more effective to look at some simple system designed for teaching. For example, Oberon project from N. Wirth at ETH.(http://www.projectoberon.com/)