I've attempted to level fingerboards before (and have a Squier Jaco-style jazz with an ebanol board that needs sorting out, although putting flats on it calmed it a bit).
But it is a pain as it requires:
- strings off
- string tree off (otherwise the block can knock into it)
- nut out (to get the area around 1st position correct)
- truss rod slack (to make sure you are not sanding in back bow)
- body protecting or neck off (neck on and you knock into the pickups
at the body end, neck off and it's like trying to sand a live fish... or get
a flat chunk of wood and screw it onto that).
If the dip in the fingerboard is localised under one string putting a straight edge on makes it hard to spot whether you've done it right, and so after you have reassembled it's hard to tell if you have done enough. It's just occured to me that on an ebanol board a dab of paint might help (sand until it is gone) even if not so advisable on a wood board as it might soak in.
I might have one last go before I take it to a luthier. My much older bass with a rosewood fingerboard could probably do with a maintenance sand too, but it is in a better situation even with a softer board. When I first got it I used rounds on it and it ripped it up, so I got it levelled, toughened it up with tung oil (real stuff) and switched to flats. It lasted three years first time round, and a decade afterwards!