small, chain
瑣 centers on things that are small, trivial, or linked together like a chain. The chain sense is a specific application of small linked parts.
瑣 combines 王 (jade) with 鎖's right-side phonetic component, which likely contributed the sound and the idea of small linked pieces, like a jade chain or small jade ornaments.
The left side is jade 王, and the right side looks like a chain of small parts. Imagine tiny jade beads linked together into a delicate chain—each bead is small and trivial on its own.
For サ, picture a small chain of jade beads being sold at a Saturday market: Saturday -> サ, and the tiny linked beads are the small chain.