It should really be a personal choice for you I think, given you don’t mention absolute financial limitations that can force limiting to one. If in your heart your want two - that it sounds like you can afford, but not without the regular finance impact all families make with several children - you’ll probably make it work fine and be happy. If in your heart you wish to stick with one, just do so.
There’s absolute immediate financial limitations (it doesn’t sound like you’re in that category of strong worries about how you could keep afloat, so you’ve done the responsible rational thinking part it seems), and then there’s trying to predict the long term future, which is imo pointless.
I say this as someone who with no prior respiratory etc issues very much almost departed in ICU due to a pneumonia complication last year, at 31. So it has changed my outlook on life a little. As you’ve done the necessary responsibility of being a parent, ie analyzing your finances, go with your heart, life is way too short and unpredictable.
A second perspective : I do have a sibling 7 yr younger, practically no contact but irrelevant to you I think, as I was taken into care at 10 by my grandparents (who were absolutely lovely, despite not having much funds, still saved and took me to Paris Disneyland!). I really didn’t mind siblings or not (was still in contact with mine and have been since recent years) or money or not, it’s the love and care that matters. You could consider at that age I was a sort of only child (2 different continents, sibling young), and they made efforts to foster friendships, which luckily I didn’t struggle with, sleepovers etc, so I didn’t feel lonely.
Sorry that was a bit long, it’s very cliché but the future is unpredictable, as long as there’s health above all, and love, and an element of rationality because bills need to be paid… only my perspective though☺️. My grandparents are both departed now, and I so cherish those years, the overwhelming memory isn’t outings, it’s love and safety.
Best wishes in your choice.