IT WORKED !!! I fully uninstalled dhcpcd, re-configured my ethernet adapter, now the resolv.conf is correct and stays correct after boot, dietpi-config correctly says that Cloudflare DNS is used, and requests to bt4g (and all the other non-working trackers) now correctly redirects to the website !!!
Jackett now works perfectly with these trackers.
Thank you so much for your patience.
I really don’t know why dhcpcd was installed, I don’t remember having any app needing it…