I did the Remembering the knaji program years ago and recommend it.
There is a site called “readthekanji.com” that will really help with learning the readings. It doesn’t focus on onyomi vs kunyomi. It gives you a sentence with a certain word highlighted and asks you to type the reading of that word. Sometimes it’s the onyomi that gets used, sometimes it’s the kunyomi. That way you are focussing on patterns and learning reading in context, not just trying to remember the readings in isolation. Also, it presents the words in such a way that they are grouped by words with similar kanji, just like RTK groups knaji with similar radicals.

The down side is that it costs $5 a month and if you fall out of the habit of studying and forget to cacel your subscription, it just keeps charging you. It would be wise to set up a reoccurring reminder in your calendar to cancel your subscription (or pay close attention to your bank statement) just so you don’t forget for too long…like I did.

How much Japanese do you know, outside of the kanji? Do you know a large amount of vocabulary already? Or are you just beginning? Also, what are your plans for studying the rest of Japanese?

I ask because the worth of this technique varies depending on the other techniques being used:

If your plan is to memorize all the kanji meanings, then memorize all the kanji readings, then memorize vocab lists and grammar rules, you’re going to spend months studying without ever being able to apply your knowledge. That would frustrate most people.

If you plan to memorize all kanji meanings as a foundation, then focus your further studies around topics, then I think you’ll have more luck. Knowing the kanji meanings can help you remember compound words (volcano = fire + mountain). Things like readings and vocabulary and grammar all depend on the context of the conversation, and there’s very little rhyme or reason to them at times, so there’s no point memorizing them all at once.