What is the lightest wood, suitable for a bass, that doesn’t compromise the sound/construction? I guess this would include fingerboards, as they also contribute to overall weight.

The lightest body woods I’ve used are Paulowina, Spanish Cedar, and the super light varieties of Swamp ash. They all work, but yield a softer overall attack. I’ve had my best tonal success (while keeping weight down) by using alder or hemlock. For fingerboards; the softest/lightest I’ll use is Indian Rosewood…..and that is rare. I gravitate towards more dense woods for fingerboards for both tone and functionality.