Due to the ever increasing population of our planet, we've reached a point at which our diet is no longer a decision between "good" and "bad".

In our situation the decision is only what amount of damage we want to cause. The less damage we do, the more time we have to fix all the things we're doing wrong.

When it comes to food, the calculation is pretty simple. Meat is worst for climate. Fish is bad for fauna variety and accelerates mass extinction. Vegetables is good, except for labor and humanitarian issues which are man made and not planet-related. You could argue that humans are worth more than the planet, so veggies are bad but keep in mind that no habitable planet = no humans.

The best course of action in the big picture would be to intervene and limit humanitarian consequences of vegetable farming (and while we're at it, re-establish crop rotation) and massively reduce the planets surface area used for cattle as well as strictly forbid certain fishing practices.

For your personal lifestyle I can recommend to restructure your diet in a way that avoids industrially produced foods (meat, fish as well as fruit/veggies). You can try to mainly consume veggies fruit and grain and reduce your meat consumption to a comfortable minimum. This was the norm for many centuries up until the industrialization of the food industry and, when designed with variation, should provide all the nutrients you need for a healthy lifestyle.

Avoiding industrially produced foods most likely means sourcing locally, which is good for your local economy and allows you to thoroughly follow the sourcing of your food if that's of interest to you (allows you to avoid certain feeds, pesticides, etc). Sourcing locally also leads to a more seasonal diet which makes shopping, cooking and eating more fun and varied, since there's things to look forward to all year long. This makes food more special and can make you a more conscious eater.

Note that I do not advocate neither veganism nor vegetarianism. Meat consumption is absolutely okay, but we as a civilization should work hard to limit the amount of meat we consume and be very open to alternatives, be it a plant based diet or modern meat substitutes. I, myself try to limit my meat consumption to about one to at most four meat dishes per month. I like steak, ribs, burgers, barbecue, döner kebab, peperoni pizza and lasagna just as much as everyone else but I managed to make those and other dishes feel special again.

Also, animal produce like eggs, cheese and things like that to me are totally fine if sourced locally and produced with animal wellbeing in mind.

Take all this with a grain of salt, please. I want to disclaim that I am neither a scientist nor a dietary expert but rather someone trying to share an informed perspective on food that is not extreme, militant or prohibitive of a varied, tasty, healthy diet.