There’s a script for self-study ([Userscript] Self-Study Quiz) where you can review the radicals/kanji/vocab on your own. There’s also an option to filter for leeches (items you got trouble remembering even after multiple a couple reviews) if I remember correctly. Though as @seanblue said don’t do those right before a review. But there shouldn’t be a problem when doing those directly after you newly learned them or after a review.
For a quick review after learning new radical/kanji/vocab you can also look at them again on the summary page right after the quiz and see if you still remember everything correctly.
The reccomendation for 15-20 lessons (read: words, not sessions of 5) is also mostly so people don’t get overwhelmed by reviews later on when items come back for master/enlightened/burned levels. If you feel like that’s still too much for you to remember, go slower (my maximum amount for kanji is 10). The focus is on quality not quantity. Or as someone else pointed out in another thread: it’s a marathon, not a race. You don’t have to finish first, just focus on finishing at all.
Also this: