Hi @Eric_S
I believe you are correct. My understanding is that timers are not active during system_off. You would want to spend most of the time in system_on sleep which would happen if you pend on a semaphore, use suspendLoop(), or spend most of the time in the delay() call.
I was seeing a 12uA increase in power usage when using attachInterrupt. That would explain at least some of the 17uA increase you are seeing. You could save this by using the custom interrupt code posted above. However, is it worth it for your use case? With a 18650 and 38 uA of current draw, I think you get many years of battery life. Extreme power savings makes more sense if you are using something like a coin battery.
If you are looking for extreme power savings, you might look at your circuit and sketch for your initial case. You should be able to get an initial baseline of < 5uA (I saw 3uA) with the LED off and in a delay() call. Maybe bluetooth isn’t fully disabled.