Thank you for sharing such great information .

it is a little surprising cp210x converters are out of stock, these are 'common' items. i'd guess it is mainly due to the 'chip shortage'. hopefully, as things get alleviated, they could become abundant again. A thing though, i'd guess for MCUs that is intended to be connected to the pc, it'd be better to get those that has on-chip usb support. these days a lot of MCUs such as stm32s, RPi pico, esp32?, and various other have built-in usb support. Those are better as native on-chip usb means that your 'baud rate' is effectively 12 Mbps (well the host does the polling, so it is multiplexed, it won't be 12 Mbps all the time). And moreover, using native usb takes one beyond usb (cdc) serial. One could for instance implement usb-hid, usb-audio, usb-mass storage, usb-video, fake a 'printer', the list goes on for all the usb device classes and beyond. native usb is the 'serial' of today, pretty much, it is 'virtual' comm port after all. on a different note, for chips without 'native usb support', the uart pins rx, tx, gnd, vcc could go to the headers. then just about most usb native microcontrollers, can double up as a usb-serial converter (e.g. with a sketch, i did that in the 'old' (libmaple) stm32duino core github.com/rogerclarkmelbourne/Arduino_STM32/tree/master/STM32F1/libraries/A_STM32_Examples/examples/Communication/USB-uart-w-signals this is quite an interesting topic in a sense that linux has configs to tell it (modem manager) that it isn't a modem at the other end, to prevent it from trying to 'reset' it. grin. That old 'modem' is today substituted by all the mcu using it (usb-serial) in its place.

One thing I'd love to do is a fairly large USB PD power pack, and there's only one IC on Digikey designed for power delivery... The other piece of the puzzle is a battery management PMIC, possibly one that can handle a bunch of batteries in series.

I see CH340G chip on many boards. Checked recently fried board of my 3D printer, that too happened to be this chip. Quite popular it seems, I wonder how much it costs.