We have introduced a 5 month old cat to our 2yo cat, then a 3.5yo cat 12 months later. A friend told my my first cat was lonely because she talked to bugs and birds and snuggled with our dog when he was inside. They have the occasional quarrels but nothing nasty and are more regularly cuddled up or grooming each other. I love it and would do it over. It would likely change the dynamics of your house but maybe in a good way.
We also now also foster kittens. To help with any anxiety from the introduction we always put an anxiety collar on our girl who is more prone to anxiety.

It could be loneliness. You could try fostering another cat or kitten, see how that goes and if it goes well, maybe you can look at something more long term.

I would agree with maybe looking at adding a kitten, so your cat doesn’t feel like she is in completion. We made the mistake of adding an older rescue to our mama and our younger girl cat was not happy. They viciously fight so have to be completely separated for their safety which is a pain and means we have to actively switch them around from the cat room! I would also consider how safe the kitten would be though with a dog that doesn’t really like cats.

Ooohhhhj, a good question. I’ve never had to deal with this. Good luck with it all

If you’re to get one, maybe a kitten? That way your older cat won’t feel intimated.