Well spotted Jane. I bought the Waveshare SPM so that in future I can upgrade to larger solar panel - and forgot that my current panel is below its minimum. 
I’ll go back to the Sunflower SPM 5V with solar panel and connecting 5V Out to the microcontroller board - but I’m not confident that the SPM will operate as expected without its own LiPo battery. I’m afraid I will need 2 LiPo batteries if I want to use the UM FeatherS3’s LiPo connector and battery voltage monitor with a solar panel. Overkill.
But I just came across a forum post on DFRobot which made this click for me. As I understand it, the answer is to connect the Sunflower SPM’s battery connector to the microcontrollers Battery connector. The microcontroller can read the battery voltage, and the SPM will do the solar charging.
