I've just started to get into microcontrollers, but I have been playing with boards based on the NRF52840[0]. I'm using them to build an air quality monitor to measure CO2[1], volatile organic compounds[2], temperature and humidity.
I'd recommend the Adafruit NRF52840 Feather[3] or if you want a smaller version (that doesn't include a LiPo battery charger) get the ItsyBitsy nRF52840 Express[4].
I haven't used the Arduino or CircuitPython programming environments, I've just been using Arm GCC and Nordic's SDK.