If the lack of open hardware bothers you, there are several non-x86 architectures that are fully open source. Chief among them is RISC-V[0]; SiFive[1] is taping out RISC-V processors to real hardware, selling dev boards, and stuff like that. These are real, competitive, Linux-capable CPUs, although widespread adoption has not happened yet.
On the more minimalist front, the ZipCPU[2] is simpler (though has less software support), and it's author writes some really great blog posts about HDL[3] (Hardware Description Language -- how these CPUs are defined).
You might also enjoy developing for very small embedded systems. I like AVR CPUs such as the ATMega328P (of Arduino fame) for this purpose. Although not open-source, they have quite good and thorough datasheets. AVRs are very small 8 bit systems, but as a result, you can fit most of the important details in your head at once.
If you want to talk more about these topics, feel free to get it touch with me. You can find my contact info on my profile, and on my website (also listed in my profile).
Pretty much all inside it is open for you to look into and change as needed.
At some institutions they teach how OS work by students implementing Pintos, a full x86 OS.
In my first year I had to build a full raspberry pi emulator from scratch and then code an assembler for it. These programs would then run natively on the raspberry pi. People wouldn't even touch the hardware until much, much later into the project when the emulator has confirmed to be working.
I disagree with the view that you need a full FOSS hardware to understand a platform. You can do a lot with a VM/QEMU.
Also they seem to have released a cheaper micro ATX board: https://www.raptorcs.com/BB/
Gen Z here. I tried to give manufacturers the finger and try to play with systems level software on modern PC's and ended up retreating after realizing, among other things, just how much modern computers are not for hackers. Everything feels like a "undocumented" (by which I mean the documentation is likely only accessible to OEM's and those with large business deals woth them) mess. Even when it is available, I'm too scared to touch it. I found something I wanted to try to to hack around with till I saw the license agreement I had to read in order to access the docs. I couldn't parse out the legalese, especially regarding open source works and I'm not consulting a lawyer to figure out if I can post some project I might hack together online under an OSS license. The few things that do get open sourced are incredibly half assed and you can still tell they're designed for corporations, not hackers (i.e. edk2)
Conversely, I have an old IBM XT compatible luggable sitting in my closet. The manual for that has the full BIOS source llisting. Nowadays I mostly just hack around woth old non-PC devices, but for the most part computing just isn't fun for me anymore.