Malaysia is not necessarily more threatening than Indonesia so it’s rather unclear why you mention it in particular. Indeed, Indonesia bombed various things in Singapore during the Konfrontasi period, whereas Malaysia has never done anything like that. (Well, I’ve heard rumours of actual hostilities about some islands supposedly from a relative in the army, but either way both sides have kept it very quiet if that did happen.)
I don’t really see the point of an iron dome system for SG: Singapore can hit KL or Jakarta in a matter of hours (if that), their relatives shop and children go to school at international schools in Singapore, there’s a big American base, if Singapore gets hit they’ll immediately end up in a terrible recession, etc.