Koe no Katachi, 3-gatsu no Lion Recommendations
Deals with a depressed kid and really conveys the emotion fantastically
Both are heartfelt dramas that deal with depression, isolation, and the human condition. They both teeter between heavy and uplifting atmospheres and give off a similar vibe. Season 2 of 3-gatsu also touches upon bullying in a similarly beautiful manner as A Silent Voice does.
If you liked A Silent Voice, you'll probably enjoy 3-gatsu and vice versa.
Both are very personal drama's, that give the viewer credit to infer and which focus on the protagonists emotional turmoil. There isn't a shred of doubt in my mind that 3-gatsu no Lion is the most similar anime to Koe no Katachi in existence. Trust me, I've seen everything considered even remotely similar.
Koe no Katachi - Definitely watch it subbed
3-gatsu no Lion - Preferably watch it dubbed
Both of these animes deal with the consequences of depression and, while to a much less pronounced extent with 3-gatsu no Lion, bullying and feelings of inadequacy. The primary driver in both of these stories is character interaction, which both helps and hurts the MCs.