It's way more sustainable and ethical (and possibly healthier too) to take b12 supplements (even if they're not sustainably produced) than any amount of animal flesh (other than roadkill I guess).

• b12 deficiency is widespread even among meat eaters
• b12 is generally supplemented in animal feed or the animals themselves are supplemented. Skipping the middle man is clearly more sustainable.
• If the animals are not supplemented, you can't know if you're avoiding deficiency. Furthermore a tiny amount of a vitamin (milligram or microgram order of magnitude) takes way fewer resources to produce than 500g of meat/fish.
• no matter how "humanely" they treat the animals they still get killed, unlike with the supplement. For dairy and eggs the slaughter is more indirect but it is still a fact that animals in the industry end up getting killed.
• In terms of sustainability, it doesn't matter that much if a product is local, the most impactful thing is the mode of production or the product itself (https://ourworldindata.org/grapher/food-emissions-supply-chain)

I agree with your overall point that from a population perspective it's way more practical and impactful to change as many diets as possible instead of focusing on changing diets to 100% plant-based. And I think we should focus on that, broader society interventions instead of advocating for individuals to change their diets.

But if someone who reads this is facing the choice of taking a supplement versus some animal flesh, there is no ethical or sustainability grounds for choosing the flesh. Also probably no health grounds unless there are compounding issues (eg deficiency + allergies, interacting conditions, etc). I focused on B12 but this applies to many nutrients.

It's just not true that a tiny amount of animal flesh will spare you deficiency. It's also not true that deficiencies are "tough to avoid" on a plant-based diet, especially protein deficiency.