All I am trying to do is isolate the various devices from each other…
I am running a GL-AXT1800/Slate Plus as my main and a Beryl MT-1300 (non AX model) as a remote AP. (I would like to add 1 more remote AP - whether that is another AXT1800 or an Beryl MT1300, it does not matter).
It is possible but not with the utilities the GL firmware gives you out of the box. You will have to get into luci and OpenWrt in general.
Since this is far beyond normal usage I would recommend you to play around and try to understand how OpenWrt works and what to do. Without understanding of routing, firewalling and wifi configuration in OpenWrt this will nothing you can do.
Yeah, it’s possible. To build off @admon 's point: you’ll also want to get into VLANS for the various device subnets… so you’re going to be getting familiar with LuCI real good, OP. Keep log entries of your changes & always have a bail out/restore point:
On the main router > luci > create a new VLAN, for example 10, and attach the guest network interface. Make this guest IP interface of the main router xxx.xxx.xxx.1. and DHCP enabled
Then connect the next wired router, set the same VLAN number (10 in the example) and also connect the Guest interface, but set the IP of the Guest interface xxx.xxx.xxx.2 and the gateway and the DNS server xxx.xxx.xxx.1 and DHCP OFF and this will make the main router provide IP to all devices connected to all routers.
With different SSID you can do the same with other new VLANs and new interfaces and DHCP enabled on main router and always disabled on next connected routers and gateway and DNS pointing to main router.
Choose the main router with the best processor and RAM.