Forgive me for saying ... I might be completely wrong... And I want to respect your life choices...
But when you said vegan I wondered how you get your B vitamins. I was already low so I had to give up becoming one to save animals. I gave low thyroid & read that soy mimics strogen & slows down the metabolism. So I am supposed to avoid soy to not let it slow down my thyroid...
I had convictions & lifestyle ruin my life before. I know how gard it is to do a radical change. I wanted to study medicine but had to turn my back to be saved by dirty farmers with nutritious crops... Changing my values was hard. But I would have died otherwise... To get a radical life change we need to make one... I also gave up loads of other stuff... Becoming an atheist was also mind shattering... But showing the other cheek was torture... I had to become evil to fight evil. Still working on it. Good luck finding a new path. How do you get B vitamins?
It's simple, vegan multi vitamins. There are also vegan B12 alone. I've never been deficient in any vitamins. There are vegan supplements for everything, including Omega 3.
I also have low thyroid, T3 to be exact. I love soy, and it hasn't affected my thyroid. I do take a very low dose of liothyronine. I'm also on bioidentical hormones that are 100% plant-based.
There are so many vegan products that don't contain soy, you can have great food and get all the nutrition you need without affecting your health negatively, and actually improve it.
If you really care about not contributing to the extreme suffering of non-human beings, you can choose to do so.
I too am an atheist, but I may have mentioned that in my post. I still live by the 'golden rule'. I would literally die (even before I was suicidal) before doing everything humanly possible to not contribute to the enslavement, exploitation, rape, torture, and murder of sentient beings.
My values and ethics are who I am. They all align. Nothing could make me change them. But, that's me.
At least now you know that you don't need to be deficient in B12 or anything else to be vegan! I wish you luck in making lifestyle choices you can feel good about, whatever they are.