Most of my diet is plant-based, growing some myself and trying to buy season/local, but I also include fish and fish oil for health reasons, as well as butter and eggs. I make sure the fish is from sustainable fishing, eggs and butter are from local animals eating organic, etc. I've been doing this for about 20 years.
I applaud anyone's effort to try to be more sustainable and avoid causing unnecessary suffering to other lifeforms, as well as to try and be healthy. At the same time I think a purely vegan diet is very hard to be healthy specially long term, and I find it very contradictory when some people are eating vegan but depending on mass produced products coming from another continent, wrapped in so much plastic and what not.
Everything in life has trade-offs and there is no such thing as 0 impact. I think the distancing of consumers from the process of killing the animals and over-consumption of meat connected to the meat industry (and conditions of those animals) are very harmful, but I also think hunting, raising your meat source in small scale or fishing for food when you do it within limits and with respect to the animal life, can be honorable and sustainable. Practices such as regenerative grazing can also help the top soil and are arguably more sustainable than monoculture agriculture which can feed some plant-based diets.
Diet is also very complex and being healthy has several different layers. Some nutrients can act as anti-nutrients when in certain combinations, and the same food can be good and bad in different aspects. Red meat is nutrient-dense but can be associated with cancer, fish meat can be very healthy in some ways but some fish like tuna can accumulate heavy metals. Plant diet can be great but the lack of b12 or heme iron isn't, and things like phytates binding to minerals aren't either, or soy's estrogenic activities, etc...
And then we have to consider that we humans often can't really feel the effects happening in our bodies, and we may feel good or bad with a certain diet but that does'n t necessarily mean its being healthy or not. It' s a very complex multi-faceted subject and it's important to keep this in mind when talking about diets and making decisions for oneself.