To start, I remind you I am in the US and so there are differences in treatment, and am aware that retesting b12 during treatment is considered unnecessary and unreliable.
I just wanted to share a statement my GI doctor made. After my initial 6 loading doses, my b12 level was about 790. 7 weeks later (no injections) it had dropped to about 400. When I followed up with my GI after a few injections once a month, I told him I felt an increase in neurological symptoms. He said that while numbers are well within normal range, he thinks the drop may mean that the injections are probably not often enough. He has seen that many pernicious anemia patients (or otherwise b12 deficient) with neurological symptoms don’t function well at levels below 500 with symptoms returning when injections are spread out. So he increased my injections to make sure I stay above that threshold in the future and if GP does retest to make sure it isn’t under 500.