CAX80 modem/router with deco x55 3 module mesh system in access point mode

I wouldn't shut off the wifi on the CAX. The CAX has higher capability in terms of wireless than the X55 does. 

The x55 will give you better overall coverage but the devices closer to the CAX will potentially have a faster speed and it'll take a load off the x55. 

What I'd do is make sure the CAX and the X55 are using separate ssids and on different wireless channels to prevent interference. 

so go into both and put one of them on channel 1 for 2.4ghz and the other on 6 or 11. Then set the CAX using the DFS bands for 5ghz so it has full 4800mbps link speed. Whatever section of bandwidth you use for that, put the x55 on a different one. 

That'll reduce the interference so they don't cause issues with each other. 

Then you can put the devices that aren't mobile and are close enough on the CAX. Again, it'll have faster speed and it'll take a bit of load off the X55 as its only a dual band mesh system and not a triband. Its a solid system but isn't going to have the same peak speeds of the CAX. 

That should give you the best performance.