Regarding DNS -- read up on the way that dnsmasq handles multiple DNS servers (read the dnsmasq upstream documentation). In OpenWrt specifically, I don't recall if there are options that are only available at compile time, or if all options can be implemented within the config file. But in short, dnsmasq can operate in a few different modes:
- send to only one DNS server normally; subsequent DNS servers only used as failovers
- send to all; use the fastest response
- And I think it also has a round-robin mode, possibly with weightings.
https://openwrt.org/docs/guide-user/base-system/dhcp.dnsmasq
I'm not actually an expert in the details of dnsmasq, but you might find this interesting: