DD-WRT is good, but personally, I think OpenWRT is a little better. Check both to make sure your specific router is supported. I’m using a Linksys MIMO router in a 2,800 sq ft house, but the router is centrally located and covers the entire house ( and yard ).
As for which router to get, that depends on your needs. If you can centrally local the router, and, it can provide the coverage you need, a single router solution might work.
I’d warn against the ethernet-power adapters, as you have to run encryption to keep your network safe. I’ve heard stories of others being able to see into their neighbors network via the powerline. Running encryption destroys the performance on the pwerline adapters.
There are several good routers out there. Netgear being one of them.
For a firewall, I’d recommend OPNsense ( a fork of PfSense ). I run OPNsense on a 4-NIC NUC ( with Intel 1GB NICS ).
With a 4-port switch on the Linksys router and 4 ports in the OPNsense router, I’ve got options.
I segragate my network into three main subnets. 1) IoT for Roku, Firestick, Chromecast, etc, 2) for phones, iPads, and workstations, and 3) my lab. Three separate VLAN’s where I control who talks to who through the firewall in the Linksys router and the OPNsense firewall. There are two ways into my network and both are behind a firewall.
I originally did this as part of a security project, but I don’t mind the extra protection it provides.