The Mayfly 1.1 is great for powering options – but it does mean understanding the tradeoffs.
For an FTDI cable, for debug monitoring of the Mayfly this is what I do – https://github.com/neilh10/ModularSensors/wiki/Test-Equipment-FTDI-cable
I also cut some pins to make it a “monitoring cable” that won’t cause a reset when the FTDI USB is plugged in to the monitoring computer. But only do that when you follow how the download works.
When programming the Mayfly I always use the USBC cable, and insure I disconnect the FTDI monitoring cable – as technically they are doing the same job and may fight (electrically speaking)
I would think +12V could also be provided to the Mayfly 1.1 Solar2 socket. All possible options – but only if you follow how the circuits work. Using Solar2 would allow the USB-c to be used with the terminal.
I use the board with a solar panel, and so it always has a LiPo 4.4Ahr battery plugged in. On the test bench with LiPo I don’t always need the external power, unless the battery needs charging. 🙂