How does MaruMori handle kanji?
I think RTK has definitely been good for me. I was doing Genki stuff and common words in Anki for a while before I tried it. Building up kanji knowledge the RTK way helps with not getting similar characters confused near as often, and not having to copy every time I write one.
You can get a lot of milage out of just reading the first bits about the method if you don't want to go all in on flashcards.
But getting that strong familiarity with kanji makes it easier to learn new words and such, so earlier is probably better if you want to do it.

It sounds pretty good. As long as it does have a systematic way of approaching Kanji, I think that's what counts.
Heisig's way is similar but different. It's just kanji only, no vocab or readings. And since he thinks you need all the common kanji anyway, it's done purely by components with no care for how useful the character is. The building-up-parts and mnemonics stuff I like, but I do think the ordering could use a little more consideration.
The Kohii app is actually a Foss thing that's not officially associated with the book or author. You can customize all the keywords and you record your own stories (or use ones shared by the community). I think it could still be useful for tracking characters you know, even if you're already deep in another app. (Then again, you've got a system and it's working, so no need to overcomplicate things and spend time on tools you don't need.)