@jcp77 

Sorry this took so long here is what I found out 

Yes, you can use the firmware designed for hardware version 3.0 with your version 3.6 units. Here's why:

The hardware version displayed as 3.6 is equivalent to version 3.0. TP-Link operates multiple manufacturing facilities, and the manufacturing location of a product can be determined by its sub-model number. Items with versions ending in "6" or "8," like V3.6 or V3.8, are produced in one facility, while products without a "6" or "8" in the number are manufactured elsewhere. The sole distinction between these products lies in their manufacturing origins; all hardware and software components are identical. For instance, V3.6 is equivalent to V3.0.

This means the firmware for V3.0 will work perfectly with your V3.6 units since they share identical hardware and software components.

Remember you will want to use one of the deco units with hardware version 3.6 as your main unit as i stated here once you upgrade to the beta firmware. Here is a link to the post containing the beta firmware.

Firmware 1.4.0 for Deco XE75/XE5300/XE75 Pro V2/V3 Adds Wi-Fi Access Control, WireGuard VPN & More! 

Thanks 

  @HelpFixDecoApp,

Yes, it should be fine.

The only time that chart starts to get confusing and needs looking to further is when its the second digit (ie v1.28) as these have historically been more likely to use a different version or not be compatible.

Beta firmware can be a bit different, but typically they lean towards being more 'universal' than not.

I believe that the systems should also prevent you from applying an incompatible firmware outright - but we never want to rely on that with in-progress builds and firmware.

Also, the manufacturing location does affect the hardware version, but it is not a catch-all rule; there are several outliers that break it, making it far less reliable than you would expect.