oak used for charcoal
檪 centers on a specific type of oak tree, particularly the kind whose wood is used for making charcoal.
檪 is a phono-semantic compound: 木 (tree) gives the meaning, and 楽 (raku/gaku) likely contributes the sound. The exact historical development is uncertain, but it is a specific tree name.
A tree (木) that produces music (楽) when burned? Imagine an oak tree whose wood makes a pleasant crackling sound as it turns into charcoal.
For レキ, picture a lucky oak tree that always gets chosen for charcoal: lucky -> レキ, and the tree is happy to be useful.