Is it worth it to put my cat through another surgery & chemo for mammary cancer?

My 8 year old cat was diagnosed with carcinoma two weeks ago after I found a 2cm x 3cm lump on her belly. Initially x-rays were positive, they showed no metastasis to the lungs or lymphnodes so they went ahead and did a chain mastectomy on her right side. That was last Tuesday and she's been healing well since.

They did a biopsy of the mammary chain and a lymph node that they removed as well post-surgery, and I got the terrible news today that it has in fact spread to her lymph nodes.

I now have such a tough decision to make and I would love the input of any vets who have dealt with this before.

Untreated, they estimate that she has about 3 months left to live, which is absolutely gut wrenching. Her behavior has been totally normal - curious, eating, drinking, playful, etc. So just thinking about how quickly it seems like things are about to turn is terrifying.

The other alternative is aggressive. They would do a chain mastectomy on her other side in probably 2ish weeks (from my understanding you typically wait 3-6 months between surgeries, but because it's so aggressive she obviously doesn't have that long to wait). That would be followed by chemo. They said with all of that treatment, that would give her an extra 6-9 months. Best cases scenario is that she could get an additional 2 years, but they made it clear that that is the exception, and is typically not the case.

I'm debating whether or not it's worth it to put her through the second surgery and chemo. It breaks my heart to think of doing nothing, but my biggest concern is what her quality of life would be for that extra 3-6 months if I do go through with the surgery and treatment. Like if her quality of life is going to be pretty low, it feels selfish to put her through that just so I can get a little more time with her...

Also the surgery is obviously very invasive, and I feel awful putting her through that again just so she could spend another month of her very limited time left recovering. That said, she did handle the first surgery like a champ so I'm hopeful that it wouldn't be too terrible for her to go through again.

A few questions:

How effective is chemo typically for this type of cancer in cats - and what are the symptoms of the chemo? In your opinion, is it worth it? Or is the discomfort from the treatment pretty evident/does it take a toll on their quality of life?

Has anyone experienced that 2 year success story? Specifically after the cancer has metastasized and you've gone through the whole surgery/chemo treatment. Just wondering if there's any hope.

Do you agree with the 6-9 month prognosis being typical?

And just in general - what is the cat's quality of life post-surgery/treatment? Do they seem to do okay, or does the chemo really limit their quality of life?

She's acting completely fine right now, which breaks my heart in its own way. It seems like if there's a chance I should do it, but I'm terrified that I could put her through all of that treatment and end up spending her final months with her in discomfort, in and out of the vet, etc....maybe it would be better to just pamper her and not try to fight it any further.

Any advice is welcome!