Animal cruelty

Do you mean the firstborn of a pure animal? It is not sacrificed on the eighth day, but within the first year of its birth. As it is said (Deuteronomy 15:20): “You shall eat it before the Lord your God year by year” (see Mishnah Bekorot 4:1). Perhaps you meant to say that the year begins on the eighth day after the birth of the firstborn, since only from that day is he worthy of sacrifice (see Leviticus 22:27).

1. Once one waits long enough, there is no tseva’ach here, since animals do not remain with their parents when they grow up. It is true that the giving to the priest must be done first (after 30 or 50 days in the case of a thin and coarse animal), and perhaps this time is also sufficient (I do not know the nature of these animals). And this time is not intended to be saved from tseva’ach, but rather to not burden the priest (see Mishnah Bekorot 4:1). And perhaps the reasoning for tseva’ach also exists here, but because one waits anyway for this reason, the question of tseva’ach does not arise anyway.
2. The question of veganism arises in many contexts, not just this one. In fact, there is no problem with this, since the sacrifice is eaten for the priest and not for the owner. Only if the priest is vegan will there be a problem. But what will you do with all the other sacrifices that are eaten for the owner? What will you do with the tefillin houses that are made from animal skin? And the books of satam that are written on leather parchment? I have no solution to all this (and I am also not bothered by the use of animals as long as there is no torture or suffering involved), and we will have to wait for the Sanhedrin to decide.

Rav Kook writes that in the future, no animal sacrifices will be offered, but only plant sacrifices, and that will solve the entire problem. We’ll wait and see.