> Most legumes have a pretty high carb-to-protein ratio, and are also incomplete proteins.
This is a red herring, based on misinformation about nutrition. It is not hard to fulfill your daily protein needs and get in all your essential amino acids and maintain a healthy calorie intake while eating a diet of legumes, grains, and vegetables. It can be done for a fraction of the price of a omnivore's diet and can be a lot better nutritionally than downing tuna every day (i.e., there's more opportunities for variety).
> there's probably no reasonable definition of sentience that includes all animals and excludes all plants.
There are a few, but it actually is largely irrelevant. If you stop eating animals you also consume less plants. Every time you eat an animal you have to consider that in order to raise that animal it had to eat a lot of plants that you could've been eating instead. Raising animals is massively inefficient energy-wise, which is why our plant consumption goes down when people stop eating meat.
So really there's no need to draw a "sentience line" in the sand. Assuming everything is sentient, vegetarian diets are better for both plants and animals. But even if it were necessary to draw this sort of line, it would make a fair deal of sense to say that anything without a nervous system is at least relatively less likely to be sentient. There are some arguments for plant sentience due to their limited molecular signalling responses to harmful stimuli, but there is definitely a lot less evidence to support that hypothesis than there is to support the hypothesis that tuna are sentient.