Human sacrifice seems to arise via very specific social circumstances. Namely research has found that the more stratified a human society is, the greater the likelihood it will practice intentional human sacrifice. This is mostly done for several reasons, most notably creating a greater social divide between classes and serving to keep the masses in line through fear and terror. It is also more likely to arise in societies where the worth of a single individual is very low, and people are seen more as a resource than groups of individuals with unique value. E.g., one thing I've heard is that Mesoamerican societies developed a more extreme sacrificial tradition in part because in the absence of pack animals or other labor-saving devices, it was more useful for rulers to treat their subjects as menial laborers rather than being of value for individual skills. Other cases are known, for example Carthaginian leaders were known to sacrifice their own children during times of crisis as a demonstration to the people that the elites too were suffering from some crisis.
Human sacrifice throughout history is typically not consensual, using the Mesoamerican city states as an example yes there was a lot of social pressure treating sacrificing ones' life to the gods as a noble goal but at the same time the people being sacrificed were often prisoners of war. When human sacrifice was at its height, under the Aztecs, city states were forced to participate in butterfly wars (basically mock wars where the Aztecs captured would-be sacrifices) or else the Aztecs would march in and take people as sacrifices anyway. Butterfly wars, widow burning, the sacrifice of children by Carthage, the infrequent mentions of human sacrifice in the Greco-Roman world (mostly the Minoans, early Greece, and the Roman Republic), sacrifice in Austronesian cultures, etc., are almost all characterized by the person being sacrificed not wanting to die. In this sense it's almost less sacrifice in a traditional sense (that is, the willing relinquishment of something for later gain or a greater good), and more of ritualized murder.
In fact, speaking of widow burning, history gives a good example of why these kinds of practices would not survive in a modern setting. As most probably know, India was conquered and colonized by the British (which as you mention are the main inspiration for Ishigan culture). The British found out about the practice of widow burning and were horrified, and banned it across India for being immoral. Same with the Spaniards and Mesoamerican sacrifice, to the point that most Mayan and Aztec books were burned because the Spaniards thought them to be evil upon seeing how widespread Mesoamerican practices of human sacrifice were. Depending on how the Ishigans run their empire, it is likely they would do the same. The banning of "barbaric" practices by a conquering power is written in the biographies of rulers around the world throughout history.
Human sacrifice also tends to fall apart in societies that value critical thinking or have place value on individual merit and have some form of class mobility. Mostly because someone will start questioning the system and notice that human sacrifice doesn't work unless the gods themselves appear in the flesh and clarify that they demand it. In which case, again, its debatable whether it's really a religious ritual, or no different from a tithe of human lives being demanded by a sufficiently advanced being, because it's a coercive act not based on faith.
It's hard to see how that much blind faith could be maintained in a space-faring setting (your not-Aztecs have to maintain their spaceships too) unless you're going for full Warhammer 40k in which the barbaric maintenance of blind faith and anti-intellectualism in the face of a space-faring future is part of the point. Either that or your not-Aztecs don't actually build their ships but use slaves to do it, but then you get into the question of why a slave doesn't just invoke Niven's law on the not-Aztec homeworld and sterilize it. Regardless, a major difference in your setting is unlike the Imperium of Man in Warhammer 40k, the not-Aztecs have competing human ideologies to deal with, who demonstrate very well that one does not have to perform sacrifices to the gods in order to thrive.
Critical thinking destroying human sacrificial traditions isn't just a Western thing, Chinese administrator Li Bing eliminated the sacrifice of young maidens to a river god during the conquest of Sichuan pretty quickly, and it's thought he did so because he figured out the people involved weren't doing it to appease a river god, but rather because they just wanted to get rid of their daughters they didn't want to take care of.
