Honestly, I think it's selfish to have a kid with someone in his mid-50's, and people who say '' you want one so have one'' are being very very unreasonable. It's human beings we are talking about who will be brought into a world and situation they have zero choice over, that being said, you already have one so I would commit to having two because being an only child to an elderly parent is hard and will probably mean they won't feel like enjoying their 20's in case something happen to their dad.
I am biased because my mom had me young and my grandparents were 40 and 42 when I was born so about 12/15 years younger than your husband when I was born and it was brilliant having young grandparents with lots of energy etc... but I am in my 20's now and my grandparents are in their early/mid 60's and the decline in their health and energy levels is so clearly visibly apparent, it breaks my heart every time I visit them how old they have gotten in comparison to the memory I have of them 40/early 50's. I have young cousins and young siblings who have a totally different relationship to them than I had because they just don't have the same energy level anymore, and I am forever grateful that I got to see and enjoy them at their ''peak''.
Honestly, the only thing that make seeing them get older and slower each year more bearable and less heartbreaking is the fact that they are my grandparents and that it's the natural process of life and that I am actually lucky to be having grandparents in their 60's when my friends have grandparents in the 80's that are either dead or dying.
I would be absolutely devastated if they were my parents because the difference between now and a couple of years back is so visible that you can't help to think that it's only going to get worse and it is worrying, to realize that sooner rather than later I will lose them and I am in my 20's/they are in their 60's, your husband will be 75/77 when your second child is 20 (depending on how long it takes you to conceive) and I am sorry but it's so unfair to bring in a child knowing that, if their parent is still alive by the time they reach 30, they probably will be in a care home or might suffer from dementia and other age-related illnesses. Sure one can lose a parent at all age, but there is a difference between actively bringing a child to life in a situation that isn't ideal (that's what my mom did when she chose to have a kid as a teenager) and bringing in a kid in optimal conditions and life throwing an unexpected kick at you.
The relationship I have with my grandparents would be very different than the one I have with them now if I was 10 vs my current age. They are both lovely but my grandpa, who used to be incredibly sportive now has bad bones and is much slower on his feet, he doesn't hear as well so I now find myself having to repeat stuff and say them louder than I normally would, he has also become very stubborn and opinionated to the point that he is kind of unbearable at times picking fights over nothing (you can agree with him but if you formulate it in a slightly different way than he will pick a fight thinking you are disagreeing), he is also completely out of touch with my generation let alone the generation of my 12 yo sibling.
My grandma has slowed down a lot too, she doesn't have the same patience she once had and can be slightly more irritable, she is now deaf in one hear so communication is also a struggle and requires a lot of patience/repeating. She is slightly less out of touch than my grandpa but still, I would really struggle to connect with them if they were my parents.
Ultimately it's your choice, but I don't think any child would pick to have an elderly dad/parent in their 20's if they had the chance and that should be reason enough not to proceed imo (though again better to have a sibling in this situation than none so in your case I would probably still go for it for the sake of your dd).