The instructions on that page work just fine with fw4. As in the necessary nftables rules are created with those same instructions.
Otherwise I’m not seeing the benefit in using dnscrypt-proxy over simple stubby. With stubby requests also go via dnsmasq and sent out via port 853 using DNS over TLS. I use the Cloudflare Family DNS over TLS resolver. Stubby is supposedly faster than using DNS over HTTP.
See:
https://openwrt.org/docs/guide-user/services/dns/dot_dnsmasq_stubby