This app was mentioned in 12 comments, with an average of 1.50 upvotes
小学生手書き漢字ドリル1006 and Kasui.
(Disclaimer: I wrote Kasui =)
If you're in 1st or 2nd semester or equivalent Japanese, AND you are using other study tools, AND you just like using it on the bus or something? Go for it.
Otherwise it's a waste of time.
This is kanji drill app is great: https://play.google.com/store/apps/details?id=jp.co.gakkonet.quizninjategakikanjidrill&hl=en
I second the 小学生手書き漢字ドリル suggestion, also available for Android: https://play.google.com/store/apps/details?id=jp.co.gakkonet.quizninjategakikanjidrill
I like it so much I also bought the paid version: https://play.google.com/store/apps/details?id=jp.co.gakkonet.quizninjakanjidx
There is Kanji Drill 漢字ドリル. It covers the first 1006 characters (elementary school levels). You can click on the Answer button to see the character and trace it (but you won't be awarded points).
Kanji Training 漢字トレーニング picks up the rest of the Kanji characters until the end of high school years.
The apps are designed for Japanese students so there are no English explanations. If ads annoy you, you can get the paid versions for ¥400 a pop.
​
I would maybe look into this app and other apps made by the same company. This one is made for Japanese kids and is designed for Kanji Writing Practice, but it tests your knowledge of phrases as well.
Till now I've been using Remembering the Kanji Vol. 1 and 小学校手書き漢字 app to learn the kyoiku kanji and that's just enough for me. It doesn't really matter what books are you using, but the general suggestions to learn Kanjis are:
I am currently using 手書き漢字 and I really like it. It has two versions, one to practice elementary school kanjis (教育漢字) and another one to practice high school kanjis, so you can practice the whole 常用漢字.
What I like the most about the app is that you are presented with a short phrase in hiragana and you must replace the highlighted kanas with the corresponding kanji (which you have to write by hand). I think that's pretty good because that way you don't have to memorize the sounds for every kanji in an unnatural way; you have a context that helps you remember the different pronunciation for every kanji.
I wouldn't really consider them an immersion resource, they're for writing practice. Things like this
For games, I suggest Koe, Japanese Dungeon, and if it's still available (it had gone away last I checked) "Survive! Japanese"
I also like JapaneseClass for quick bits of practice
Would it be worthwhile starting RTK or KKLC as an intermediate learner in order to improve writing kanji?
I'm actually quite good at recognizing a kanji's meaning and reading (I've burned nearly everything in wanikani, if that's any sort of worthwhile metric, and not long sat N3) but I really struggle to produce the kanji I need out of thin air on a blank sheet of paper. I do really poorly on the Kanji Kentei DS game and this 小学生手書き漢字ドリル1006 app on android, and I feel that neither have been particularly helping me that much.
There's some nice kanji drill apps aimed at students you can practice with on the go. I've been using these ones recently. I like it as you can press the display button to show you the stroke order to write over if you don't know it, but also practice from memory once you get the hang of it.
https://play.google.com/store/apps/details?id=jp.co.gakkonet.quizninjategakikanjidrill&hl=ja
https://play.google.com/store/apps/details?id=jp.co.gakkonet.quizninjajhkanjikaki&hl=ja
https://play.google.com/store/apps/details?id=jp.co.gakkonet.quizninjategakikanjidrill&hl=ja
I used this for memorizing the kyoiku kanji. Good recognition and easy to use.