Who says that B12 supplementation works less good than eating meat? The last studies I read on supplementation all showed good results. Better results than among omnivorous people.
That's why I say a plant-based diet is science experiment with no plausible basis in evolutionary history. I'm not saying it's bad per se, I'm saying we don't actually know if it's good.
Looks like you have done your homework and are not in fact deficient, at least in terms of those biomarkers and reference values. A lot of plant-based dieters are not that diligent, affluent or educated.