Will using smaller kmers help get larger contigs? If not, then what?
You can try s-aligner (free for 15 days) which usually gets quite larger contigs and quite larger NG50 for metagenomic data containing viruses (also phages). Indeed metaSPAdes also gets larger contigs, despite shorter than s-aligner.

Ref. s-aligner: a greedy algorithm for non-greedy de novo genome assembly