My note: 'Low stomach acid' and 'microbiome issues' seem to the culprit in low B12.
Perhaps, excessive intake of antibiotics in childhood may have caused low stomach acid and microbiome issues in your case.
So, antibiotics overuse might be causing lot of problems which are not immediately visible. Like so many things in medicine like wrong treatments for squint, appendicitis, heart diseases, etc., perhaps taking antibiotics was also based on bad science.
What could cause malabsorption of iron and B12 on a meat based (+dairy) diet?
Any ideas?