If the processor doesn't sleep very soundly, or if there are other circuit components that have the same problem, then it seems you could use an RTC module or similarly efficient tiny processor, powered by its own coin cell, to periodically turn on the power to the main circuit instead of just waking it up. Then the main processor would maintain the power on while it's doing its thing, then shut off the power. The power switching circuit would just be a P-channel mosfet, an NPN and a couple resistors, and use one GPIO pin. And then you wouldn't really have to worry about sleep current or a regulator's quiescent current or current used in dividers since the whole thing would only be powered up briefly.