While there are still daily reviews to be done on RTK1, there will be no more adding new cards, and the shorter reviews will leave more time for new learning resources. A few posts on the Koohii forum discuss the next steps, along with many other blog posts. Read this one, it gives a good idea of the options available. After quick research, I set up for either Core2k 6k from Smart.

RvTk (Reviewing The Kanji, and sometimes Koohii) is an invaluable website where students can share their Kanji stories and mnemonics. Perfect for the lazy and less imaginative, as I am. But once you finish RTK1, you might be tempted to switch to a more comprehensive SRS tool, like Anki. Among others, it provides online and offline reviewing, mobile apps for reviewing on the road, and many options to fine tune the SRS.