[Black Leviathan]

One of the game world we play in most frequently with GURPS is Fading Suns. It's a high-magic setting blasters and laser swords and psychics and it very blatantly has zombies and priests with blatant occult spellcasting. It works because it is very Science fantasy, you can play and technologically proficient character with all of the technobabble that explains how a ancient alien gate propels your starship faster than light but there are legit malevolent beings that inhabit darkness and bookish priests that can blast things with flames from their bare hands with the right incantations and gestures and it's just accepted.

I think you have to have a little bit of that doubt that science is any more real than magic in your setting. Magic wouldn't work so well with a hard cyberpunk world, but it fits fine in Shadowrun where ancient spirits are running wild and nobody is completely sure why the super advanced technologies work.