When I stumble upon this ayah, I noticed that the Bahasa version translate it literally as 'healing for sickness in the heart', but actually Allah didn't limit it to sickness only here. The language suggest that the Qur'an is a healing for what is in the chest, which makes the scope of healing a lot broader. It could include things like sorrow, sadness, grief, fear, emotional pain/scar/injury, and many more! And indeed I have experienced Qur'an's healing myself.

Notice also how Allah doesn't use the word Qalb or Fu'ad, which means it heals more than just the heart, but whatever is in the chest. So there must be more to it.

Allah also didn't limit the ownership of the chest with 'your chest' or and simply stated 'the chests' (plural), which could imply that the Qur'an can heal not only mine as I recite it, but also others that hear the recitation too. The number of chest being healed is larger. Therefore, when we recite the Qur'an at home, everyone in that home would benefit from it.

Allah addresses the mankind, a broader audience, in this Ayat, not just the believers, not just the muslims, not just the muttaqin, not just the muhsinin, but the whole mankind. So This could means that the healingof the Qur'an could have an effect for non-muslims too.

THEN, Allah limit the next part about the guidance and mercy aspect of the Qur'an for only the believers. This reminds me of a Youtube comment under a Qur'an recitation video about a non-muslim that enjoy listening to the Quran for a long time to calm his heart, yet he is not a muslim still.

SubhanAllah, how precise it is the word of Allah Azza wa Jalla!