You might be interested to know that, as luck would have it, this was the first issue I picked up when scrolling through looking for a good introductory task to get used to the project: https://github.com/LemmyNet/lemmy/pull/3168
So this might change in the not-too-distant future (I haven't started doing any frontend work to support it yet though).
I might be missing something, but is it the case that we won't be able to discuss veganism? I'm vegan for ethical reasons so I do get where you're coming from with this, but I think there might be perspectives you're not seeing.
I became vegan something like six years ago and prior to that I was vegetarian for my entire life. I was vegetarian because I had a moral problem with killing things. I became vegan because I eventually ended up actually listening to vegans' arguments for why dairy etc. aren't hugely different morally speaking from meat. If you want to create more vegans - and thereby reduce suffering - vegetarians are probably some of the most receptive people to these points, assuming they don't get alienated.
Assuming I'm not missing something and it is the case that we'd be able to have friendly discussions with people about why we believe veganism is a better moral choice, I don't really see what's so wrong here.