I never used Kamma for these as I know well position of the suttas.
Just that for people basing morality only on Kamma, might be missing something useful.
A girl drowning in a river, one doesn’t have bad Kamma for not saving the girl for one has no intention to kill the girl, the girl is already dying due to other causes. Yet, most of us would say it’s heartless to not do whatever we can to help save the drowning person, if we can swim, and the current is not too strong to harm us, etc.
Sure, attaining to enlightenment is better than saving a drowning person. But then, how much time does saving that person takes away from the practise?
While on the path and after attainment, it takes about equal amount of time eating plant based food only vs eating meat + plants. It’s just the initial mental effort to do the switch, which I assure people after many years of being vegan, by now it’s almost effortless for me, more like natural. Arahants still has habits. Perhaps it’s easier to cultivate the vegan habit before enlightenment rather than after.
So the morality to consider here is greatest good for the greatest number, relatively. There’s no provision in the suttas that one must eat meat. There’s no provision which prevents one from going vegan. So there’s no conflict of deondology vs utilitarianism like in the trolley problem.
Thus going by for the benefit of others and oneself, which is environmental sustainability, able to face oneself when watching slaughterhouse videos etc, it’s logical to go vegan.
And your bugs get killed by plant harvesting thing still gets addressed in the same manner as in the previous discussion of what if plants are sentient. More plants gets harvested to feed animals than for humans, thus animal husbandry has more blame to bear. Minimizing it is still a good thing to do.
And for the health thing. If one is seriously facing the problem of iron deficiency and cannot get enough iron even after eating a lot of vegan food, then there is legitimate reason to resume eating meat. To use that as an excuse, turning a blind eye to the numerous other health benefits of going vegan doesn’t seems like being sincere about concern for health. If ever I heard of vegan facing nutrition problems, lacking iron is so not one of the problems. Whereas for meat eaters, we see heart disease, cancer etc…