Fingerboard levelling Questions

So...
As some may know, I have a bass I am almost finished converting to fretless. The 24 fret ebony board has been defretted and maple veneers installed in the slots. Some fingerboard sanding has been attempted, then stopped when uneven results became apparent.

I have acquired some DIY tools, partially based on reading many threads here and elsewhere, and partially based on my recent unemployed status.
What I have, so far: an 18 " aluminum carpenters level, a couple of machinist's rules, 3 strips of granite roughly 2 - 2 1/2 inches wide in 7, 8, and 9.5 inch lengths, and some 2x4s I can cut to whichever length. Am attempting to find a suitable surface to utilize to make a radiused block.
Various threads suggest such tools can get the job done well enough, but definitely a lack of consensus looms.

Also, no real agreements re: specific sandpaper grits and types (start with 100, start with 400, sand 'til the surface is nice, sand up to 3000 with wet paper, aluminum oxide is best, 0000 steel wool is just as good, etc.)

I may be looking for hard and fast guidelines in a place too tough for rules, but is there a straight-forward process here? Looking for a dead-level board, no epoxy or coating.

Would probably prefer the radius be saved, but flat fingerboards are new to me. Anyone offer pros/cons re: flat fingerboards?

Sorry for all the questions, anything helpful to offer would be... helpful as well as greatly appreciated.
Thanks!

 

So, in the spirit of anecdotal information:

The patient: 24 fret ebony fingerboard, fret slots filled with maple veneer. Plan is to remove veneer sprout and level the board, also removing veneer buzz. Before I removed the strings, I marked the string paths on the fingerboard using a .05mm pencil and a machinist's rule.

I started with 10in and a 12in radius gauges, 9.5in x 2.5 in piece of granite, and white 400 grit AO paper. Applied paper to granite and sanded using even strokes, following the string paths, moving slowly.
Took approx. 15 mins using light to moderate pressure, wiping dust away as needed and checking radius.
When smooth, I burnished/polished with 0000 steel wool for ~5 mins or so.

Also knocked off the nut using an xacto knife to score the original finish, and a piece of wood and a screwdriver to apply the actual knock. Placed 400 sandpaper on a larger piece of granite and patiently sanded the bottom of the nut to lower everything evenly.
1 trial fitting and some more sanding, done.

Finished board is dead smooth, evenly level, and has a slightly compound 10in to 12in radius. Nice, even notes across the board, no veneer buzz. It needs a new battery, I think, and that's it. Oh, and maybe a bridge.