The number of sitters who apply probably are narrowed by each factor or complication involved in a sit. So say someone didn’t have a problem with being away less than five or six hours (many of us don’t mind), but many sitters don’t know how to (or might not want to) administer shots. Or they might be worried about sitting for a dog with health issues, because they might get sick and leave the sitter with extra care, maybe even leave them housebound.
In your situation, I’d suggest posting your sits well in advance and considering what elements you can control to make the sits more inviting. That could better your odds for finding a good sitter. And with an unhealthy or elderly dog, personally, I’d especially want a good sitter.
You can link to your sit in your bio and various folks on the forum can share feedback on how to potentially make your sit more appealing: How to add a listing or profile link to your FORUM profile.