That is the extent of his regular shinra tensei in his full body. He doesnt have zero chakra lol he had a low amount maybe but not zero. No matter how low it was it doesnt make a difference for a simple shinra tensei. deva path produced the same regular shinra tensei when he was at the beginning of his fight against naruto as well as the end. his regular shinra tensei stayed the same. A regular shinra tensei doesnt take much chakra for nagato and relatively stays the same power whether he is low on chakra or has a lot of chakra
This a point where he had a lot more chakra before using it to fight a lot before fighting naruto and the destroying the village
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This is another regular shinra tensei when he was much lower on chakra
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still around the same power. Also like i said before one regular shinra tensei uses a very minor fraction of his chakra so having a lot more chakra won't make much of a difference unless we are talking about a chou shinra tensei which uses a lot more chakra and a lot more powerful
But what we saw is a simple shinra tensei it doesn't get any more powerful than that