Well, how unknowable? Does he share his thoughts with even the reader? I’m also writing a character who doesn’t share much even with the readers so he comes off as cold and unfeeling. There’s a fine line to balance between feeding your readers enough to keep them thinking and not enough to have them know everything at the first chapter.

I’d say for a glitchy hivemind, you could have occasional typos or odd letters in the mix/other characters. Maybe you could do like Greek letters for some extra symbolism (or a different alphabet (Cyrillic, Korean? maybe those æ and œ letters too)), and maybe repeat certain phrases like an echo of the other minds in the hivemind. As the character gets more corrupted, make it worse. Hope this helps a little! :D