I don’t think it’s ever too late to start WaniKani. We have some members who have reset and are doing their 3rd or 4th pass through the whole program just to keep the readings and meanings solid in their memory, as well as to practice pronunciation (particularly with pitch accent, and there are some great plug-ins for WK you can install, such as one that maps the pitch accents, very helpful if you’re a visual learner).
To me, the real question is, is it worth it for your budget? Even at a breakneck speed you can only finish WK in somewhere around a year-ish (most people take far longer than this). You can pay monthly or buy a lifetime subscription. They always have really good sales at New Year’s, so unfortunately it’ll be a while before those come around again. If you can afford WK, I recommend it. WK is not ordered by simplicity of concept, but by simplicity of the kanji visually, so you might learn some new things even from earlier levels, as some kanji with more “abstract” meanings (that might be taught later in Japanese schools or other learning programs) are more simple visually and will be presented sooner.
I second the notion that it’s nice to learn kanji for words that are usually written in kana alone but DO have a kanji. It seems helpful to me, and it’s not like there are a ton of those words so I don’t feel they really slow down the learning.
First 3 levels are free, so in any case, finish those before you subscribe and use the time til then to decide, or you’re just wasting money. Those levels are mostly numbers and really simple kanji you’ll easily recognize, so you might as well get them out of the way.
EDIT: I also HIGHLY recommend setting a 12-hr schedule daily to do WaniKani, as it will speed up the process and let you review items 3 times on the first day you learn them, which really helps retention in the following days. E.g. learn your new lessons for the day at 9AM, the reviews come 4 hours later so come back at 1PM to do those, then they will come back in 8 hours, so come back at 9PM to do those reviews. There are more in-depth posts about this method here on the forums, with tips about which items you should be doing to best use this method, but it will make things go along at a muuuuuch better pace than if you simply check back twice a day or at random times. What times you pick daily is totally up to you as long as the first and last are 12 hours apart, with the middle session 4hrs after the first, but sticking with the same times every day will help you get the later reviews done consistently too as they come back on 12/24/48 hour (etc) intervals and will always be there to do when you come back to your schedule.
Learning this method drastically improved my WK experience.