Don’t really know if it is normal or ideal, but in my personal usage, during large review sessions (60+), I can reliably get around 95%. For small review sessions (30 or less), I typically ignore the percentage.
As for whether you are “undermining the system” by doing extra reviews with physical cards, it’s not inherently a problem. The idea behind the SRS is to make your learning more efficient by minimizing the amount of times that you review items. However, the intervals they’ve chosen may not work for everyone; I have personally begun taking an extra, manual review session in between Apprentice 1 and Apprentice 2, and that has aided my retention pretty well.
One thing I would caution against, though, is using your physical flashcards for items that are Guru or above. Since those ranks are more about checking whether you can retain the information over time as opposed to learning the material for the first time (or learning it over again; it happens), you may do yourself a disservice by doing extra reviews in between WaniKani’s scheduled reviews.
I personally don’t see a problem with learning ahead so long as you learn things in the intended order (radicals, then kanji, then vocab). Just be sure to pace yourself and make sure you’re not overdoing it. I would recommend using something like Reorder Ultimate 2 if you wish to advance more quickly in WaniKani.
As for using an Anki deck to learn the meanings first, try it out and see if it works for you. Just make sure it’s a deck that only has the meanings WaniKani cares about. Otherwise, you would be investing time learning extra meanings that may not be useful to you.
Hope this helps!