Yes, it's 3.6v for the LiFePO4 cell (I'm using one of these instead of a Li-ion battery because I want to run the Wemos D1 Mini without a power-sapping voltage converter). Agree on reducing the charge voltage, I was going to go for 3.5v but used 3.6 to get a higher charge rate while testing - interesting you suggest 3.45v, I may give that a try.
My next option is to use a simple relay to completely disconnect the battery from the LM2596 module unless the module itself is powered, but that feels like a horribly clunky solution! It will also consume a bit more power, but at least it won't discharge the battery.
Many thanks,
Tim