@Shamar run a local resolver, and have it do the DoH. Bind (9.17+), Knot (4.0+), and Unbound (11.12) all apparently support it. Configure this resolver via /etc/resolv.conf, NetworkManager, etc.
Unbound is pretty lightweight, I would look into that first.
If the debian packages are too old, look at running it in a podman (or docker) container.
That what I did understood by reading the doc.
#PowerDNS's #DNSDist seems a superb project, just maybe a bit overhelm for what it should do in this specific case. Yet seems likely the only option, actually.
It's a bit of a shame no other #DoH #CGI have been written in compiled languages so far... which is likely why few resolver support such forwarding.
When I first read about DNS-over-HTTP, I saw it as a further centralization attempt by #Google & friends (which sadly includes @mozilla these days), since to get a working DoH service you need good sysadmin skills and a stable public IP: not something a kid with a cheap shared hosting can set up.
And ə PHP implementation would be too slow.
#FossilSCM made me realize that a simple CGI in C could have good performances and be widely distributed, so I wrote one (still early alpha).
Now I can use it in most (non enterprise managed¹) browser, but I'd like to try it system wide.
Anyway... thanks for your help guys!