I tried a Flip organic "insert" which is more like a cross between a prefold and a flat, with a thirsties hemp doubler folded up inside it and a fleece insert on top with a Flip cover and it worked! That's as brave as I've got yet when it comes to prefolds ;) it didn't leak though!
My baby is only a month old but I just use an OsoCozy with a Thirsties duo wrap and no problems so far. I did try a Gerber prefold I was given an it was a total fail.
For years and years I didn't know there were any options outside of prefolds and PUL covers nor did I know any folds aside from the trifold. The trifold held up fine for us at night for the most part. You could wrap a Hemp Baby or Joey Bunz Premium insert inside a prefold or flat to add more absorbency. For flats you could fold two flats together for extra absorbency. Some people pad fold one flat and lay it inside of a 'fancier' folded flat. Others pad fold a flat and wrap a prefold around it. There are lots of options.
I haven't used wool, but from I read about it I believe that would be a good extra layer for your heavy wetter.
I have a night time system that works and uses prefolds and sometimes flats, but in a pocket -- I like the stay dry layer the pocket provides. It gets bulky, more so than with my bamboo and microfiber inserts, but it works.
God-loving, Hubby-loving, Peanut-loving, people-loving, stay-at-home Mom, total nerd, and major tightwad