I am running an Archer AX21 and a number of extenders in OneMesh mode.  I am not thrilled with the coverage and speed of the network with the extenders (currently RE220s with 2 RE300s ready to install) and am upgrading some of them to faster extenders.  But I think a second router with its advanced beam forming is really the way to go (or possibly a Deco system).  The issue with the Deco system is that I am fairly sure I will need three due to the roofing arrangement on my second floor blocking the highest speed signals between rooms.  The highest speed Deco systems are only sold in twos.  So my next step will be to install another AX21 I assume in Access Point mode.  However I have two questions, when I first tried to setup my AX21 in AP mode I got an IP conflict with my Wifi Cable Modem/Router.

Questions:

Do I need to configure a fixed IP address for the Router running in AP mode outside the DHCP pool range? 

Or do I not connect the router running in AP mode with the regular Ethernet ports instead of using the Wan port? 

How is the best way to configure the second router as far as channels, SSID, and password to make it easiest to operate clients? 

Is there a way for clients to do smart roaming between routers like in a Mesh network? 

I am reluctant to spend above $400 USD (for two nodes) on Deco X90 then try and get a third node when I know AXe devices will be coming soon and to maximize my network I would prefer those.  Also, my backhaul is an issue since my home wiring is current Ethernet Cat 5 (Not 5e) and I have not yet snaked any Cat 7/8 cables just run a few along the wall.  So I am thinking two routers are a good interim solution until the AXe Mesh devices are available.  I am running a fair number of smart home and entertainment solutions so a strong single network is mandatory although most of the smart home devices are OK on a slower strong network.  Traffic is somewhat high given lots of 4K video as well as multiple security camera videos.  I also run a backup server but that traffic is typically late at night.  Internet is Gigabit and I am moving above Gigabit on my home network probably to 2.5 Gbit in the near term.  Replacing the Ethernet cables is also a near term project so any backhaul should be wired fairly soon.

Maybe I should have spread my questions over several posts but they are all related.