The next few stages are not going to be terribly entertaining. As noted, I need to get the pin mappings into Marlin 2.x, which will produce an absolutely *minimal* printer. I don't say that lightly, I mean it - no SD, no LCD, no...most of the pretty bits. But once that's working and in, I can switch modes. I ordered from China (and just received) a beautiful little blackpill board - that's an STM32F4 system with SDIO SD card and a FMSC LCD interface.
Just like the lerdge boards.
What this means is that I can develop SDIO support for HAL_STM32 on a system that's actually designed for debugging, then move the code over to support Lerdge.
LCD? Same deal. The LCD in question uses a different command set, but that's irrelevant - I will wire it up identically and use it to debug FMSC LCD interaction, followed by U8G LCD interaction...followed by moving the code to Marlin. THe LCD supports the exact same XPT chip, so I can even debug touch interaction.
But first, I am going to finish out the M200 work.