Glad to hear that a treatment plan is in place and you are pursuing another biopsy. With all of the new biomarker and mutation testing, it will benefit you to have a full work-up.
I think everyone here have questioned their decisions during their diagnosis and treatment as you are. So much information is thrown at you and many times is sounds like a foreign language. As you ease into your treatment plan, things will settle down, at least it did for my mom. She received the same treatment for her lung cancer recurrence that you are currently planned for. She had great success with it, like many others. She took a total of 6 Keytruda + chemo infusions and then went on to Keytruda only for about 1 year. She has had no evidence of active cancer for over 1 year (it might be close to 2 years, I need to check my math!). My mom ultimately went off of Keytruda due to side effects, but many people are able to stay on the drug for quite a long time, with minimal side effects. If this is indeed the treatment you will continue with, you are receiving the standard of care.
Here's the basics of your treatment plan: basically, Keytruda will ramp up your immune system to kill cancer cells and the chemo will move throughout the bloodstream cleaning up any straggling cancer cells hanging out (that is how it was described to us anyway!). If you have a high PD-L1 score, this means that the cancer cells are really good at hiding, Keytruda specifically removes the cancer's ability to hide so your immune system can find it and kill it. Should your lung cancer be a good fit for this treatment, you are in good hands with it (that's my opinion anyway!).
Please keep us posted throughout your journey and please know we are here to help any way we can!