Synchronized Network

Hello! I currently have multiple devices that monitor beehives temperature, humidity and weight and send the data to a central node every 15 minutes to be uploaded to the internet. They are powered by AA batteries and communicate through XBee S2C transceivers forming a mesh network with their DigiMesh protocol. As everything is battery powered, even the central/coordinator/router node, I implemented a time synchronization so all the XBee S2C are put to sleep at the same time and they wake-up after some minutes at the same time, transmit the data to the central node and then everything goes to sleep again. I later discovered this is called a synchronized network or beacon mode network.

I really liked the nRF52840 chipset and it will be great for the new version of our product. The main problem I'm trying to solve and it's stopping me from adopting this new platform is that I'm not finding a way to have a synchronized mesh network as I have now. I need absolutely all the devices in the network to go to sleep as everything is battery powered and must last a year. After reading a lot on the internet, this forum and on your amazing infocenter this is what I learn and some questions about each topic:

  1. BLE with no mesh: The maximum advertising time is around 10 seconds. So I could advertise my sensors data to the central node every 10 seconds which in the case of Bluetooth gives me really good battery life, almost the same as my XBee sending every 15 minutes, awesome, really. The only downside here is no mesh support which is kind of important for me.
  2. BLE + Mesh: The main problem here is battery life, according to your docs I can make some nodes to sleep but the central node or router can't sleep, ever. Is it possible to implement my time synchronization on top of Bluetooth mesh so I can make the central node to turn the radio off and go to sleep and wake up together with the other nodes when they are sending data? In this way I could have Bluetooth mesh AND a synchronized network for really low power consumption. I don't know if it's possible to turn off the radio and then resume the communications as if I have never turned off the radio.
  3. OpenThread: Mesh support also, no way to put the main node to sleep, can I do the same as in 2?
  4. Others?: Any other protocols available for this chip that I'm missing? I saw ConitikiMAC from the Contiki OS, the ultra-low energy mesh protocol from MiraOS (paid), GoMacH from RIOT OS and TiMAC from Texas Instruments but none of them are officially supported by the Nordic SDK so they are really the last option if an option at all.

Any help or guidance would be really appreciated!