Your partner using shadow work decks sounds pretty intense. The darker decks can be more meaningful when you set the mood - like only using one candle when you pull cards. You don’t need to use a darker deck for this stuff, but if he resonates with them, then lean into it.
I picked up the Maleficium Tarot last month and honestly wish I hadn’t. The artwork is beautiful, but there’s something about it that makes my stomach turn every time I use it. Perfect for what your partner wants, though - it might push you into spaces you don’t want to go otherwise. I personally don’t like decks like these.
Has he tried just using a “normal” deck for the same readings? You could get one of the popular decks that you can work with, too, and he can try something a bit different.