I help my grandparents out with computer stuff quite a bit, but I live far away, so I usually have to help over the phone. So having an interface that you can easily describe over the phone is pretty important to me.

When I try to sign in to most apps on my TV, it usually displays an code that you can type in on another device so that you don't have to type in a long password using the D-pad on the remote. Could you maybe implement something similar for your website? This way, my grandmother could just call and read me a code, and then I could handle the sign in remotely. As long as you only need to sign in ~once a year, this would be my preferred option.

Not all seniors have trusted friends/family who can help them, but lots do, so making it easier for the helpers will in turn make it easier for the seniors. Plus, there's no phishing risk for the senior with this method, so it's a relatively secure option too. (There is a phishing risk for the helper, but presumably they're the least vulnerable person in this scenario)