We consider serially-concatenated coding schemes over channels impaired by insertion, deletion, and substitution errors. Specifically, we focus on the interleaved concatenation of an outer channel code with error-correction capabilities and an inner marker code with synchronization capabilities. To limit the decoding latency, marker code-based synchronization is performed only once per received packet, i.e., iterations with the outer decoder are not allowed. We first numerically evaluate, through mutual information analyses, the ultimate rate achievable by this concatenated scheme when standard bit-level synchronization is performed. Then, we introduce a novel symbol-level synchronization algorithm that works on groups of consecutive bits, and show that it improves the achievable rate. Besides the achievable rate analyses, which allow us to optimize the marker code, we also report error-rate simulation results that confirm the superiority of symbol-level synchronization.