The main issue with the Animorphs series isn't the depth for adult readers, IMO: there's some genuinely affecting stuff which held up when I reread it in my twenties. No, the problem is the unevenness: the series was being published at the rate of one mainline book a month, plus the occasional Chronicles (backstory novels focusing on a specific important character or setting piece), Megamorphs (larger books narrated by multiple characters at different points rather than the rotation through the main cast of who is narrating a specific book), and Alternamorphs (noncanonical what-if CYOAs). Obviously this is a literally impossible writing schedule for anyone to meet, yes that means you Brandon Sanderson sit your ass down; most of the series, by volume, was ghostwritten.
The problem this presents is that you will have a deeply psychological book about the struggle Tobias has to set aside his trauma and work alongside a defector from the enemy who happened to have brutally tortured him for days back before she defected, wondering all the while if his suspicions of her defection being genuine are rooted in fact or emotional impulse and also needing to control the instincts from one of the most horrifying morphs yet, bookended on one side by "fuck it, we're sending Cassie to Australia, she'll be back by the end, and this story will never connect to anything or be mentioned ever again" and on the other by "we're going to use a shrinking ray to pursue Napoleon-complex aliens that are hiding in Marco's nose."