You don't see laminating filler, because filler is something you sand, laminating resin isn't something you sand... so much as grind. The gummy surface on laminating resin isn't sandable unless you mix up a real hot batch of it as a flow coat over the top surface and get a near surface cure.
Most folks would be filling the lows with mat and grinding it back until things are straight. Once it is fair, a thick rolled coat of waxed gelcoat
would cover the rest of the issues and get sanded down with 40 grit until fair and until you just can see the old high spots with 80 on the pad sander.
That skips the lions share of spot fairing. The rest, spool up gelcoat with a taste of cabosil in it so it acts like fairing putty...
For raw glass spot filling I like straight milled glass fiber. It sands and grinds the same as mat so you don't get soft spots that disappear under the grinder. 40-80 grit range just about right to get through the sticky stuff.
If you want to do it the hard way:
Talc is cheap
, easy sanding
3M glass bubbles cut 3/4 to 1/4 cabosil works and doesn't sag. Spreads like whipped cream. Acts a lot like 3M's Vinylester putty. Medium cost...
I don't like phenolic microballoons in laminating resin as it seems like it they have a tendency to stay gummy the whole way through the filler and not just at the surface. Most expensive.
Don't use cotton flock or wood fiber if you can help it. Cotton flock is hard as nails, wood fiber takes on water
You've also got a minimum thickness that you can go with MEKP cure polyester. Most gelcoat is promoted hard enough that it will cure in thin film, polyester laminating resin doesn't always cure out in the thinnest of films, particularly in high humidity! Waxed will... Unwaxed, not so much. Remember that your "thin" skim is an 1/8th inch thick. BPO cream cured fillers go down to a feather edge and cure out in thin films... Part of the reason they exist.