Here is it is! My idea, implemented 1500 years ago. It is called the Thousand Character Classic now. It is a Chinese text written with 4 characters per phrase. Each character is used once and makes a total of 1,000 characters. This is just what I want, except I want one for Japanese, using modern characters and a total of about 2,000.
With something like this, you could practice 8 or 9 lines a day and go through all of it each month, keeping your writing abilities fresh. That wouldn't take up very much time. 9 x 4 = 36. So for 2,000 characters, just 72 a day keeps the Kanji doctor away. That would surely be less than half an hour. I would need a recording of it and then each day I would listen to it and write out the portion for the day.
Here is the Chinese Wikipedia page called 千字文 containing the 1,000 character text. And here is the Japanese page on the subject of 千字文. And here I found the text with a Japanese translation of each line in hiragana 千字文全文.