Thanks, everyone!
I gave her Miralax yesterday and today and she has had two good poops! In the litter box, even! She feels fine and is eating well. I mixed some warm water with her food today (and for the other kitty too) and they both seem to like it so I will keep doing that.
She turned 18 today and we had a great party with catnip!
My “younger” kitty (11 1/2) is having no problems but I will probably go on mixing her food with water too since she is not exactly a kitten.
To also share things I saw online: giving a little milk can help with constipation ( (but I don’t want to give any more of this because I don’t want a UTI). A few greens can help (green beans or peas or kale or spinach – my girl ate a little bit of spinach today so I may use that as a fallback), or some canned pumpkin. I would rather go natural than medical as I do not want her to become dependent on a laxative, but as long as the Miralax is working I will stick with it, since so many of you recommended it from your own experience. I certainly won’t mix all the above!
My younger kitty drinks from their bowl; older kitty will only drink from the toilet, the kitchen tap, my water glass, or any glass/bowl she finds in the sink with water in it.