Welcome back! 
As you mentioned, I think people generally put the end of genki II around N4. I would guess your kanji level might be close to N3 with the wk level you reached, but without a lot of reading and grammar practice you will probably be struggling quite a bit with those sections at an N3 level.
One option for trying to figure out your current level is to check out some sample test questions and see what feels obvious or not obvious at the levels you’re considering: For Examinees: Let's Try Sample Questions! | JLPT Japanese-Language Proficiency Test
Wanikani doesn’t follow the jlpt or kanken order for introducing kanji, so if you really want a focused study tool then you might want to try to get an anki deck tailored specially made for the kanji, vocabulary, and grammar on a specific level of the test.
For trying to figure out where to reset to, you could try using wkstats to see what level your items stop being burned/enlightened, resetting to that point will immediately chop most of your reviews, and then when you manage to make it through the remaining ones, most of those items will end up being burned, giving you almost a fresh slate to start adding lessons again.
There is a book club starting this month for the quartet textbook series, which is supposed to pick up about where genki II leaves off and go through most of the N3 stuff: 4️⃣ Quartet 1 and 2 Study Group [ongoing - Chapter 3], if you are in a textbook kind of learning mood! If not, then I’ve enjoyed using the 新完全マスター series, which has separate books for reading, grammar, and listening practice questions that are styled after the jlpt tests.
Depending on what kind of phone you have, there are mobile apps that let you access wk on the go yes! Anki also has mobile apps that will sync progress as well, if you choose that route. Smoldering durtles is the updated android wk one, I don’t remember what the iphone one is off the top of my head though
For commute studying, pretty much any sort of reading or listening practice will be good too! There’s lots of beginner oriented podcasts, check out some of the stuff people listen to regularly in the listen every day challenge thread for some good recommendations and ideas.
You could also try out reading the NHK easy news articles, or move up to the full articles/other newspapers if those end up being too simple.
I generally just bring my kindle with some easy manga on it to work, and that’s been helpful, I think. Check out the Absolute Beginner Book Club for some good recommendations and good resources for starting out reading native content!
Ultimately deciding between N4 and N3 is just a “how much time can I commit to studying” question, since the study-hour gap between them is fairly large. If you’re already working full time and have adult responsibilities, it might be a real challenge to push for n3 in time for december, depending on what you determine your current level is.
But anyways, best of luck with whatever you choose! I like your zeal!