I re-read your post and see what you’re saying now. I don’t know if anyone will ever make such a script, seeing as it still doesn’t exist, but I will agree with what a few others have already said.
Due to WaniKani’s order, you’re likely to see less and less kanji/vocab you already know at N3 level very quickly if you continue. WaniKani doesn’t strictly follow JLPT, 常用, or even frequency order, so it should start to get a bit more bearable fairly soon, I would think. N2 kanji start popping up a bit more around level 10, and just because vocab uses kanji from N3 or lower doesn’t necessarily mean the word itself is a common/low N-level word.
That being said, if you aren’t interested in the mnemonic system you may want to save some money and use Anki, or maybe even Kitsun if you’re really set on a quality UI. Anki is obviously free, but Kitsun has all sorts of decks for vocab and kanji, and you can skip any and every item you already know. It even has a feature to mass-hide items you learned in one deck from others to avoid double-studying.
Only reason being that WaniKani’s biggest selling point (in my opinion) is the mnemonics, which if you don’t need then maybe WaniKani isn’t going to fill the role you want as well as a different platform.