@Larrens

My suggestion would be to run ALL your Deco's in AccessPoint mode, and (unless you really need the coverage of it) turn off the WiFi on the WDR3600 and use it as just a router.

Then you can use the Deco's all as wired (if you have connections to where you want to place them), or some combination of wired & wireless.

I would suggest that you make the at least the 1st ('Master') Deco as wired for stability.

You may be interested in the setup I describe at reply #377 in this topic: https://community.tp-link.com/en/home/forum/topic/98898?page=27 (which is on page 27 of that thread).  This does not cleanly match with any of the "FAQ" I've seen so far, but it seems to me that if one has more wired devices than one has Deco's AND you already have a router+switch for them, use the Deco's as simple (non-routing) Access Points makes the most sense.

I also recently changed out the network router (my Deco's are AP mode), and that alone has greatly improved both network performance and stability.

The new router is a Ubiquity EdgeRouter 4.