My mom has advanced stage NCSLC adenocarcinoma too. Health-wise she is not in the prime of her life, but she is here 3 years later. I took the diagnosis harder than she did and took it upon myself to educate myself all that I could so I was the best advocate for her. I too started looking at statistics and felt there was no hope. I allowed myself to get lower than low. It was in one of my 2am "research" sessions that I happened upon LUNGevity and these forums. Things began to change for me and I was finally able to grow into the advocate that my mom needed and I wanted to be. You will get there too, it just takes times and persistence.
My mom's docs have never given her a death sentence and are treating her lung cancer as a chronic disease, just like her rheumatoid arthritis. At first this was not acceptable for me. I wanted a cure, I was naive in thinking that her cancer will be cured. She will never be told that her cancer is gone because it's in a spot where it's really tough to see. So we've had to accept that hearing "your cancer is stable". Never in a million years would I have thought I would have accepted hearing anything less than "you are cured". But it's all a part of living with a chronic disease and being in active treatment (my mom is on immunotherapy).
So what I am trying to say, Angelica, is that there is hope. I have found it and many of us on here have found it to. I see hope every time I look at my mom or hear her say "what a beautiful day it is today." Stick around here and you will see more hope.
As far as your question about immunotherapy, I have heard of people responding well to immunotherapy no matter what the PD-1/L1 levels are. But it seems as though your current treatment is working for you -26% shrinkage is wonderful! Lung cancer is a long-term battle that needs many weapons to be tamed. Immunotherapy may be one of those weapons that is needed down the line. But I am not a doctor. Ask you docs about their thoughts on immunotherapy for you, keep asking until you get an answer that makes sense to you.