I haven’t had an opportunity to try OpenWRT in recent times. But I remember there being a common issue with not supporting WPA3 and limited 5GHz support for 802.11ax.

So confirm that the firmware you want to flash supports all the features you want.

Edit: I’ve had good experiences with other archer routers so it’ll probably be fine.

GRC has a tool to benchmark a large list of public DNS servers and tell you the performance.

https://www.grc.com/dns/benchmark.htm

But I would expect Quad9 to be pretty decent.