I'm using DNSCrypt/doh-server as described in HMN76V's post; edited my original post to include that link.
Thank you for the pointer to EDNS0_ECS=true!
I set my wacky IP/host config back to normal so my DoH queries were showing up as vanilla "localhost" again on 127.0.0.1, set that config in my pihole-FTL.conf, restarted pihole, and... doesn't seem to work, so perhaps the DNSCrypt/doh-server doesn't support that (though it looks like it should...?). I'll try out tinkernels/doh-proxy with the EDNS config and see if that works!