Could be SIBO and leaky gut but many other things also. One possible way to test for bacteria in the stomach is by mouth throat swab. They did found a enterobacter in my mouth two years ago and another cz user reported e.coli and klebsiella.
Except the case you picked a bacteria or something else USUALLY the overgrowth comes from within the gut by gram negative bacteria.
What test did you have for h.pylori? It took me 3-4 years to discover i had it but poor performed tests overlooked it. When its chronic HP can help other bacteria to proliferate or yeast. Not saying you have it...
Another possible test is A GOOD stool exam not by usual NHS system.
I was taking acid stuff for 3 years with no problem but this spring after having too much resistant starch one time i develop massive acid in my stomach terrible hunger and so on. I was taking 3 grams of vitC without a problem now i cant tolerate 100 mg in matter of days!!!