Traveling on separate tickets always bear a certain level of risk. The key question in your case is whether ET will accept to check your bags in DAR until DPS. I have no idea about ET policies in that respect. In the western world, it is becoming more and more hazardous and unpredictable, so better to plan without. But if ET normally makes it without reluctancy, them you can book the KL flight without too much incertitudes. In that case, no immigration to clear, no bag to claim and re-check. If you cannot be sure about the check-through policy of ET or if ET flights are often delayed, then you have to play it safe and book the SQ flight at 18:20 or overnight in SIN if you want to take the KL flight but the following day to be on the safe side.