okay I tried that, using -i to specify private key. I get the same thing: blank / blinking cursor. When I use verbose -v flag, I see that in all cases (using -i, the config file, and originally) it ends with these two lines (after about 50 lines) :
debug1: Offering public key: /home/username/.ssh/id_rsa RSA SHA256:j3MUkYzhTrjC6PHkIbre3O(etc) agent
debug1: Server accepts key: /home/username/.ssh/id_rsa RSA SHA256:j3MUkYzhTrjC6PHkIbre3OT(etc) agent
where (etc) is some redacted text. It seems the server is ACCEPTING the key, which is nice. But then it's still a blinking cursor...