>Does Time Machine run on a schedule? Add a cron job (whatever the Mac equivalent is) to send a WoL ~1min beforehand?

I believe on x86 the BIOS supports setting a timer for self-wakeup. The OS can tell the BIOS "I'm going to sleep. Pull me out of sleep if the RTC passes this datetime."

Of all the x86 type BIOS and EFI, I've never seen one that didn't support that. Even thinclients and things you'd expect to leave that feature out.

I see BIOS not supporting it back in the 90's

Reverse proxy means that your pi now has to handle all the traffic for Plex and any other services. It could also make other things difficult like CIFS/whatever else you're hosting.

Depending on what Pi you're using, it's networking is over USB, and so is quite performance constrained.

Maybe use local DNS with a very low TTL. Send WOL on DNS request.

If we're talking about Plex, then local DNS is probably not going to help.

The traffic could be coming in via your WAN connection and your router is just going to forward it to an IP.

You could use DSR (Direct Server Return) so that the pi only handles the initial connection. All the video traffic goes directly from the server to you without passing through the pi.

Pi is perfectly capable of running Plex. Maybe run it on there with some cached data on the Pi and then the main content stored on the server. Then when accessing Plex, it will have all of the content showing what is available but in the background will be starting up the main server with the main content.

I guess you could just attach an external HDD to the Pi. Thats worked for me before.