@ITCCCLSUB 

From what I can understand of your situation, you have installed the M5 in AP mode because of the distance between systems.I believe that, in the situation that you are describing, you will have a much better experience with your system if you actually use the Deco M5 in a Mesh system. 

One of the general weaknesses of mesh, as you said, is that increased distance between the units will decrease the quality of the connection. However, this can easily be avoided by using a wired system to communicate between the units; which you have already set up. Most mesh systems will allow you to connect multiple units into a switch at a centralized location to avoid daisy chaining between the nodes if you are worried about latency.

When using the system as a Mesh System, the wireless range should be about the same as using the AP mode on the unites, with perhaps a slightly larger range. The advantages of the mesh systems is that the system will adjust to accommodate what Deco Unit you are the closest to and what unit can provide you the best connection. 

Here is an FAQ with some of the most common setups: Ethernet Backhaul for Mesh Systems

Here is another FAQ about the difference in features between AP mode and Mesh: Mesh vs Access Points