Did some more tests and unfortunately spinach and high-nitrate foods (all greens) continue to be out, even when separated from acid, sugar, fat and lysine.
After eaing a pound of cooked spinach pasta (ingredients: spinach, wholegrain spaghetti, vegan butter, almond milk, grapeseed oil, garlic, salt), I am still having symptoms almost a week later,
This time delay makes sense because digestion lasts for about 3-4 days, and nitrate is reabsorbed, meaning it goes into the bloodstream through the stomach and intestines and is then excreted again into the saliva after which it's absorbed again and so on.
That's why it makes sense that symptoms last relatively long.
So, sadly, I have to put nitrate-rich foods like spinach, all greens, beets, carrots etc. on the red list and continue to cook with nitrate-free mineral water.
I will check isolated sugar from fruit next. I will use dates as they are low in acid so I can separate the acid effect (seems like it's bad) from the sugar effect (tbd).