So sorry for your loss. It's always so hard to lose a beloved pet, and I think that's what makes your question so difficult. Only you know what makes sense in your situation.
To share my own experiences: I had two rescue cats, now down to one. Last summer one of them was diagnosed with a virulent, fast-moving form of untreatable cancer. I found that out during a check-up after dental work, and the vet could only diagnose after more dental work because it was a cancer of the gums and jaw. So, in 6 weeks I paid about $1400 for two rounds of anesthesia, dental work, and biopsy. Once the results came back I considered looking into treatment, but they were limited at best and it just didn't seem worth it for a few extra months. So I then spent money on pain relieving injections until it was time.
The other cat has had various weird issues: herpes infection in eyes, which then morphed into a drug-resistant bacterial infection that lasted for months; and she probably has a brain tumor that has caused seizures her entire life and is now manifesting as occasional paralysis that I treat with steroids as needed. The eye issues probably cost me a few thousand, but since they were over a period of months it was manageable. And, it was treatable and she's been free and clear and happy ever since. The brain tumor was diagnosed after noticing her paralysis. The vet hospital I took her to gave me a list of all available diagnostic tests they could run to determine exactly what was wrong. That would have cost me $3500, just to find out what was wrong. No treatment. (I happen to live in a town with a cutting edge vet hospital, which is good and bad.) In that case, I made the decision that this particular cat would be miserable with possible treatments (chemo and radiation) and that her quality of life was more important to me. And I thought $3500 for diagnosis alone was a stretch. That was over a year ago and I've had to put her back on steroids once since then, but she's living a very happy life.
Regarding pet insurance, I've decided it's probably not worth it and I assume pet care costs as part of my emergency fund. That's partly due to the fact that I own rescue cats that both came with documented pre-existing conditions. I've heard that pet insurance is very difficult to deal with in the case of pre-existing conditions.
But, all of that being said, one of the worst experiences in my life thus far was being faced with the fact that I had to make decisions about the life of another creature and I had to weigh that against the financial reality of it. That is an incredibly difficult position to be in. It really helped to think of what was the best quality of life for my fur babies, and make sure I'm keeping that in mind.