In my experience, the OpenWRT adblock package is not as powerful as Pihole. It's also annoying (involving some awk scripting) to add new lists that weren't added by the dev.

>for a second standalone device?

Please clarify my understanding:

Wouldn't the OpenWRT router be a second standalone device as well?

Are you suggesting to use a $35 as the network's primary router?

If so, do you have a model to reccomend that you've had success with and an estimate of the load it could handle?

MT7621-based devices can all handle GbE and are supported by OpenWRT. Xiaomi among others sell routers based on this chipset, but personally I'd avoid them due to their locked-down bootloaders which complicate flashing. I myself purchased the YouHua WR1200JS, which is currently available rebranded on Aliexpress for under $35 USD as model "WR330" with OpenWRT pre-installed (although you should still reflash to be safe). It saturates LAN ethernet (~936mbps avg.) in my tests, but probably needs software/hardware offloading for Gb WAN, which I think is only enabled in snapshot versions of OpenWRT.

WLAN throughput is not so good, about 288mbps at 80mhz, but this was only one client device tested and not thoroughly because I don't know enough to diagnose bottlenecks. Regardless, this is more than enough to saturate my WAN connection - no issues with 6+ simultaneous wireless devices.