Please correct me if I am mistaken, but unless there’s a local resolver for every single one of your clients, aren’t the DNS requests sent in plaintext to the upstream of your local resolver?
That might not be an issue for your situation, but I recall the benefit of something like Quad9’s offering is the encryption between client and their endpoint(s), particularly for untrusted ISP or similar.
I tried Quad9 at our business for a while, about a year. I ended up moving to something else due to latency. I'm not sure if it was a routing issue or what but there were a lot of timeouts and slow responses. I have not had these issues with other providers.
That might not be an issue for your situation, but I recall the benefit of something like Quad9’s offering is the encryption between client and their endpoint(s), particularly for untrusted ISP or similar.