You get the first 3 levels free. So I’d take that time to follow the SRS system and see how it works out. Buying the lifetime subscription now won’t motivate you to keep using the system anymore than it would buying it at the end of the year with the sale. Take these early free levels to get the hang of it, and if you hit the end of level 3 and happen to love it (which you will), then maybe buy month-to-month until the sale comes about.
I don’t want to be the bearer of bad news, (yes I do) but learning a language takes time. A lot of it. So there’s really no way around that part of it. Now how you want to spend that time learning is subject to your preference (i.e. do I spend the majority of the time on WK learning kanji, or learning grammar, or listening, etc.) but it’s still going to take a ton of time either way you approach it.
With that being said… get in line, strap yourself in the coaster, and enjoy the ride, it’s worth it.