Solved thanks to:

The problem is finally solved. Partially thanks to @petr-hejda: my self-signed certificate didn't do the trick. I had to link a domain to my IP and use certbot for a decent certificate.

The other issue was concatenating the base Uri twice inside my mint function. Corrected syntax: string memory tokenURI = string(abi.encodePacked(Strings.toString(newTokenId), ".json"));