Maybe there could be a very smart, Visser 1-like henchman who could easily defeat the Animorphs, but is held back by what Visser 3 wants to do.
If anyone ever watched the Aladdin series, it would be like Abis Mal, and that tall guy. That way the enemies fail, but they are still somewhat threatening.
Or you could just have more character deaths and whatnot. There doesn't have to be that one teammate death in the very last book. Character deaths would have been fine, and perfectly realistic, if it had been going on since the beginning.