So, to be clear, I spent my time reverse engineering software rather than the hardware itself. That said, my observation is that a lot of hardware reverse engineering is software reverse engineering since the software helps you understand how the hardware works (the Asahi guys literally built a hypervisor so they could watch macOS interact with hardware).
And software reverse engineering is just grunt work. I'd start with a very well known existing hardware platform with a very simple CPU design--the GBA is actually a really nice platform as the ARM has a very sane ISA and it's all memory mapped I/O--and get a devkit and start experimenting by writing software to run in an emulator so you can get a feel for how the hardware works.
And software reverse engineering is just grunt work. I'd start with a very well known existing hardware platform with a very simple CPU design--the GBA is actually a really nice platform as the ARM has a very sane ISA and it's all memory mapped I/O--and get a devkit and start experimenting by writing software to run in an emulator so you can get a feel for how the hardware works.