[crote]

If you're a beginner who wants to learn how chips work, something like the ATmega328 or the ATmega32u4 would by far be the best choice. It's pretty much a textbook chip, with enough peripherals to be useful yet not so many it gets confusing, and a datasheet which is quite readable.

Once you get into ARM territory it inherently gets very complex very quickly. They are massive chips made of multiple IP blocks from different vendors which have been glued together. Full documentation easily gets into the thousands of pages. The RP2040 has excellent documentation, and even that is barely enough to be usable.

With the exception of a very small group of people at the design company, essentially nobody is hand-writing assembly. There's just no point: it's incredibly complicated, and it takes orders of magnitude longer than just using the provided SDK. This makes hand-writing it only an option for hobbyists: nobody wants to pay for their engineer to spend a lot of time doing it in a worse way. Turns out "I hate cmake" isn't a very good reason to waste tens of thousand of dollars in engineer-hours.

But even for the hobbyist the SDK is probably the better choice. The one provided for the RP2040 is quite well-made, and even if you hate cmake just copy/pasting its C code into your own cmake-less toy SDK probably makes more sense than reinventing the wheel yourself.