Coming in a bit late, but as someone who has been there and done that, I think you took the right approach with your card.
As some here know, Mr. Nutrax passed away suddenly last October. He had used a wheelchair for many years, but more recently , his disabilities had increased and his health declined, so that at the time of his death, I was pretty much a full time caregiver. I have only recently begun to resume my social life and my volunteer activities.
Mr. Nutrax's birthday was only a few weeks after his death. I received a number of emails and phone calls that were all versions of "thinking of you today. Hope you are well." That was comforting, to know that people cared how I might be feeling that day.
The other day, I attended a volunteer meeting where we signed up for programs in the coming months. For the first time in a couple of years, I was able to sign up for as many programs as I wanted to do. After the meeting, another volunteer, who knew my situation, told me how happy she was to see me there. "So happy to see that you are getting back to volunteering. I understand why you haven't been around much and I know how much you missed it. We've missed you." This was a way of very tactfully saying "looks like you are enjoying yourself now that you have all this free time." The emphasis was on how much they missed me, not on how glad I must be to no longer be confined to home.
One way to say something similar to your grandmother would be to tap into the family grapevine and find out what she's been up to. Then you can call or write and say "Aunt Frieda tells me you've been visiting her/going to the symphony/working in your garden/teaching Sunday school. Sounds like an enjoyable time." If no grapevine--call her & ask what's she's been up to, just as you might with any other relative.