I am in the same boat as you. No one to take another bird and I am 72, soon to be 73. I also live in a very small apartment and there is no place for quarantine.

Bailey has been an only bird since 2021. He just sits in front of his mirror mostly. That has not changed in three years. It seems to comfort him and, although mirrors are not looked on positively for the most part in the bird world, this is a Senior bird who was never cuddly to begin with and who is (I feel) still grieving.

I allowed he and Snickers to be fully flighted and perhaps because of that neither of them were ever interested in toys. They preferred flying around my former apartment, exploring it. He still likes to chew paper, etc. He whistles short bursts to me and I answer him. That is a little routine we have. But a part of me wishes I had not gone the free-flight route so extensively because toys would be something to keep him engaged.

If I leave, I make sure to have YouTube Videos of singing cockatiels on for him and he seems to be a bit soothed by this.

Up until fairly recently, he loved to sing. When I would be coming home, I could hear him happily singing to himself. (I miss that.) That did decrease, though, when we lost Snickers. That he doesn't at all anymore has more to do with his aging than grieving.

I can tell, though, that he would love another buddy as I watch him stay close to that mirror for comfort/company (or so it seems to me, anyway).