1. SSDs (including phones) have TRIM/discard, so you need to disable it, otherwise the hidden volume would get wiped. You going out of your way to disable it is going to be suspicious.
2. Even if the above wasn't an issue, you can't really use the outer os to any meaningful extent, because you run the risk of overwriting the inner volume. That makes your decoy os suspicious. It's not definitive proof you have a hidden volume, but I doubt the authorities would care too much about that.