Another option you can have, install the cloudflared service on your pihole and use that as a DNS server. Cloudflared can take DNS requests from your clients and then proxy those requests over DoT to an upstream server which supports DNS over TLS. I have used Google in the past for this.
I had great success with this solution inside a corporate environment which blocked port 53 to all outside the network.
Another option you can have, install the cloudflared service on your pihole and use that as a DNS server. Cloudflared can take DNS requests from your clients and then proxy those requests over DoT to an upstream server which supports DNS over TLS. I have used Google in the past for this. I had great success with this solution inside a corporate environment which blocked port 53 to all outside the network.