It really depends on your own personal goals and motivations for learning Japanese. And also your approach to using WaniKani itself, given the levelling structure that it imposes.
First off, you want to consider: “Am I looking to learn as much Japanese (Kanji in this case) as possible in as short a time as possible?” And, considering that no matter how fast you want to go, knowing that learning Japanese fully will certainly be a years-long process: “Do I need to ‘go fast’ for some reason (maybe for work, or if planning to visit Japan soon), or am I okay with ‘pacing’ things for the long run?”
Having considered your own goals, then you need to consider how you want to approach WaniKani: “Am I just using WK for the short term, to learn kanji, and then never to bother with it after that?” And also your financial considerations: “Am I trying to spend as little money as possible? Or is money not such a big deal? Or am I willing to invest some up front in order to save money in the long run?”
A lot of the guides and forum-advice is centered around ‘going fast’ and ‘being efficient’, which admittedly is part of the appeal of using SRS in the first place, namely it being ‘more efficient’ by some measures.
But there’s also a risk with focusing on ‘going fast’, which is the potential for getting overwhelmed and possibly leading to getting burnt-out. Regardless of your goals, I would highly recommend taking the burn-out thing into account, and taking precautions against it, or better yet, having a strategy in mind that can adjust and adapt so as to prevent burn-out in the long run. Burn-out just sucks real bad, and will mess up any plans you have for learning Japanese.
On the other hand, if you have a good deal of free time available to devote to learning kanji (say, 1-2 hours a day, or perhaps even more), and also have strong motivations for learning Japanese, then you may be able to ‘go fast’, at full tilt, and be able to finish WK as fast as possible without getting overwhelmed and burnt-out. That’s the kind of personal story I’ve noticed that most of the ‘go fast’ guides seem to have in common. Strong motivation, plus enough free time that it doesn’t interfere with one’s life and well-being. Just put in the hard work, for about a year or so, and it can be done. But I would wager that it’s not as common of a story as just reading those guides might lead one to believe.
So, with that pre-amble out of the way, I’d say:
- If you choose to ‘go fast’, then go with the monthly or yearly subscription to WK, and follow the advice of the ‘go fast’ guides. They are good guides. In fact, even if you choose not to focus solely on ‘going fast’, reading those guides is a good idea, just to get some good tips and tricks.
- On the other side of the spectrum, however, a ‘it’s-a-marathon-not-a-sprint’ or even a ‘slow-and-steady’ approach and/or a ‘long term investment’ approach, I would suggest to strongly consider getting the lifetime subscription, so that in the long run it will be the cheapest cost, and the least ‘time/money pressure’.
If you choose ‘go fast’, then you will want to prioritize learning Radicals and Kanji first, as soon as possible, in order to unlock the WK levels as fast as possible. Use vocab to reinforce kanji, and also just to learn more vocab. Learn the vocabs whenever you’ve learned all the radicals and kanji possible at the moment.
However, supposing you choose ‘it’s-a-marathon-not-a-sprint’ or a ‘slow and steady’ approach, then you don’t need to prioritize Radicals and Kanji. You can spread them out among the more-numerous vocabs, and keep a more ‘sustainable’ balance of all three kinds of items in your Review session.
Also, for the purposes of avoiding overwhelm and burn-out, I believe it’s generally better to have a mix of things in your Lessons and Reviews, as it makes it more interesting, rather than having big blocks of mostly-kanji or mostly-vocab, which can get monotonous and/or boring, potentially leading to overwhelm.
I think the default Today’s Lessons feature is a pretty good default system for ‘in it for the long haul’ learners. This will automatically give you a mix of items to learn.
And then it’s just a matter of finding your personal ‘pace’, and adjusting how many daily Lessons you do to try to maintain that pace: Getting overwhelmed, slow down on Lessons; itching to do more, bump up the daily Lessons a bit at a time.
And always do at least one Review session per day, in order to build-up and maintain a daily habit/routine. Such a daily routine will help a lot with maintaining motivation and progress in the long term.
If you ever do run up against overwhelm or burn-out, or if you really really don’t even want to experience that at all, then consider checking out the informal thread-group 
Let’s Durtle the Scenic Route 
, where we take a very leisurely approach to WaniKani. It’s like even a step more leisurely than the ‘it’s a marathon not a sprint’ approach, and more like ‘it’s not even a marathon, it’s more like a stroll through some parks and gardens’! Don’t forget to stop and smell the sakura along the way!
