You can easily run a mesh network if your willing to run WiFi. You just need to run some cable, or you could connect them directly over wifi but you'd get WAY better results with cable. you'll have to sacrifice one of the radios (2.4 or 5) or add a USB WiFi adapter and configure it to connect to the back end. On the "Access Points" you'll configure DHCP requests be forwarded to the switch/router connecting modem (where DHCP server is). Point DNS to primary DNS server. Statically set IP of AP's (suggested they be outside the DHCP range). Disable firewall/NAT on AP's (Routers/Almond serving as AP not the actual L3 Switch/Router's Firewall). (I think that covers it)
The easier option is to get a Securifi router with "AP Mode" and it will do all that for you but might cause complications if you want a wifi mesh (Don't do it!/Use cable to connect them or suffer). I know a few other brands of AP's but don't know if i'm allowed to suggest them. A seamless mess is kinda new, but mesh networks have been around for a while.
I've configured two networks where the Almond+ was the main home automation back end and one of them it was the primary router. Both support multiple AP's which is essentially what a "mesh router" is except they make it more user friendly/plug and play. As well many are purely wifi for the back end which is horrible if you want decent performance or do gaming. they usually compensate by using a extra set of antenna on a separate network which means each router competes for spectrum with it's self and it's "friends". Meanwhile Cat6 is future proof and cheap enough, but be careful it's full copper a lot of it's fake/copper plated. Otherwise Cat5e will work.
An idea i've played with but haven't tried yet is bonding two Ethernet lines/ports together to get 2Gbps but not sure how to do it on OpenWRT.