The last paragraph about the '/etc/hosts' workaround in pfSense was incorrect; I forgot that '/etc/hosts' gets wiped periodically by pfSense. The real workaround is below:
If you have set the pfSense system-wide DNS servers to use OpenDNS/NextDNS/etc. and don't wish to change these in each individual DHCP range assignment, you can simply add 'Allowlist' entries for dns.google and cloudflare-dns.com in the web console for your DNS provider ('Allowlist' may be called something else but that is what NextDNS calls it). This will allow DNS validation to succeed for ACME. If you are concerned about clients circumventing your DNS provider due to whitelisting the Google and Cloudflare DNS names, you can always redirect all DNS traffic on your LAN to make sure it goes through your DNS provider:
https://docs.netgate.com/pfsense/en/latest/recipes/dns-redirect.html