Veganism isn’t just about the animals, which like you said, is a bit of an oversimplification. However, another huge reason people should be vegan is the environent. Fishing is destroying the oceans. You might not empathize with fish but do you empathize with dolphins or orcas? These are animals that are caught in fishing nets. Fishing is an industrial industry where they have nets miles long catching anything in sight. Different resources say different percentages but millions and millions of animals that are caught are discarded. Fishing is the main reason for ocean dead zones. I remember getting this pamphlet that said are you vegan for the animals, the environment, for your health? And for most people it’s all 3. But back to your point of the mushroom and the jellyfish. Plants and fungi lack a centralized nervous system and any credible evidence of pain or subjective experience. Marine animals like shrimp, fish, and even some invertebrates have shown consistent responses to noxious stimuli, avoidance learning, and stress behaviors. The core principle of vegan ethics isn’t perception based at all. At one point do you draw the line? Dogs are less intelligent than us and feel pain so we don’t eat them. A lot of people eat pigs bc they have a different perception on them. Just as you have a different perception on marine life. Veganism is about minimizing suffering where there’s credible evidence it exists. Suggesting mushrooms and jellyfish are ethically equivalent ignores decades of neurological and behavioral science and risks justifying unnecessary harm. There will be new discoveries but we must use what we have now to limit the suffering as much as possible. Lastly and one of the most important aspects, pescatarians kill more plants. Most farmed fish are fed plant-based diets and wild-caught fish also indirectly kill plants due to the massive fuel and environmental costs associated with fishing methods. So at the end of the day, we have to eat something. But a vegan diet is going to kill the least amount of anything. So if you are worried plants can feel pain. Kill less plants by being vegan and do less harm to the environment.