I am using NextDNS [0], which also integrates well within Tailscale across all my devices. Or are you looking for a solution that works offline within OpenWRT, without relying on third parties? It appears that there are AdBlock packages available for OpenWRT[1].

[0] https://nextdns.io [1] https://github.com/openwrt/packages/blob/master/net/adblock/...