Fairly complex sleep and timing functionality

I am writing a sketch that involves a number of timing events between the ATMEGA328 sleep periods.
In essence, the micro sleeps for most of the time.
When woken-up by the WDT, one of the peripheral modules is powered on using a MOSFET.
Various functions are performed using a timed (millis) event and once completed, the peripheral module is switched off and the ATMEGA328 goes back to sleep.
I have 3 peripheral modules connected to one UART - this may seem strange, but only ONE peripheral module is ever powered on at a given time. No two modules connected to the UART are ever powered on simultaneously. and "flow" of the sketxh
I have the bits and pieces of the sketch running and now need assistance with the timing and "flow" of the sketch.
If anyone is interested in assisting, I will be happy to reimburse for time spent.
I will also send a flow-diagram of the proposed sketch.

One thing to consider is the risk of parasitic powering of the powered-down devices through the Serial lines. This can have all kinds of unwanted effects, ranging from failing communication with the other devices down to outright breaking them.

Is there any way that I can "block" possible parasitic power to the powered-down devices?

Yes... disconnect those wires.

It's a general problem when powering down devices but leaving them connected to signal wires from devices that are still powered on!

Many devices have an "enable" input, which allows you to leave them powered on but in sleep mode, thus preventing parasitic powering. Maybe yours do as well.

For more detailed suggestions on how to set up your project you really have to post more info.

As the device is battery powered, I need to consume as little power as possible - even in sleep mode, the peripherals draw current which significantly reduces battery life.
Thus my idea of switching the “not used” peripherals OFF.
Could I possible use optoisolators on the three Tx/Rx lines - or a signal switch?

I will make a complete “package” available which will include the schematic and software “flow” should someone be interested in taking this on.
I do not expect this for nothing - time and expertise will be rewarded.

Optocouplers require some 5 mA when on, adding to your power use, though that's only during transmission. They also limit your transmission speed to 9600 or maybe 19200 bps possibly making the process slower and that way also using more power (keeping the devices on for longer).

I'd be happy to give it a closer look, see what can be done. Do post your project details!

I am putting together the entire project.to you?
Is it ok to PM

Fine with me. It appears to be a big secret, no?