And on the Vcc stuff…
I think this is the critical point I missed, and I agree it’s a great goal to be able to program it with your existing USB<->UART programmer.
I think you should be ok there. For both the ST-LINK programmers and the serial programmer the 3.3V pin is an output from the target and an input to the programmer (it’s not used to power the target).
In the case of the ST-LINK programmer, it’s pin 1 on CN4 in your schematic excerpt here. Although as you point out, on the ST-LINK strip programmer that pin is disabled (hence your n/a). On more advanced ST-LINK programmers, the programmer senses the voltage at that pin to know the target’s Vcc. I guess for the Nucleo strip programmer they can safely assume everything is running at 3.3V and it clearly works fine without it.
Checking the schematic of your shop-sold serial programmer here it looks like that 3.3V pin is how the programmer gets powered, i.e. the target board is powering the Vcc of the SILabs CP2101 via that pin.