The keisei script is really good, I recommend it. More generally I completely agree that it’s a big missed opportunity, in the late levels I would often guess/remember of a good chunk of the kanji using phonetic components. I complained about it here: It feels like WK somewhat loses its way partway through
This is a useful skill outside of WaniKani of course, for instance the other day I managed to guess the reading of 鱗 by recognizing the phonetic component of 隣.
My only guess as to why WaniKani doesn’t do that is because it wouldn’t be very useful in the early levels so they used the weird English mnemonics instead and later on it would add more constraints to their “radical” system of they want it to match the real phonetic and semantic components.