If your mppt controller doesn't have one built in (i.e. 3 sets of terminals: load, batt, pv module) I would recommend adding a battery protection relay or equivalent solid state switch between the load and the + terminal similar to in attached rev of your drawing.
Never worked with ESPs before but I'm assuming you want to use onboard wifi for power management/sleep, and to let the controller send warning that a node will shut off, that it is back online, etc. Relay state would ideally be added to the SCADA points for managing the node, maybe the protection relay even goes immediately next to the picostation so the controller can keep a log during downtime.
Really cool stuff y'all are doing. Would love to host a node but have horrible LOS at the house.