Let's explore what Dr Tim Hovanec recommends for Fishless Cycling:
The term "fishless cycling" refers to the process of establishing the nitrifying bacteria in a newly set-up aquarium without using fish which are used in the traditional cycling method. The problem associated with using fish in the cycling period is that normally, when establishing the...
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Day 1 2-3 ppm
Day ~5 1-1.5 ppm
Days 20+ 0.5 to 0.75 ppm once every few days
Day? Test for full cycling by adding 2-3 ppm ammonia
Thus, it appears one is adding on the order of 7-10 ppm total ammonia. That would give 25 to 36 ppm nitrate. Not a big deal, IMO.
Thus, I do not believe your assertion that even after a 90% water change, that nitrate could possibly be too high.