
Pnakotus Detsujin |

How much work would it take to drop Wake of the Watcher? Just go from Broken Moon to Ashes at Dawn?
Ashes is an adventure for level 11 characters, which can however be taken at level 10 if your party is well equipped against a few of the most frequent threats of said module.
If you intend to skip Wotw, i believe you could give your players like a month of downtime which allows your party to return to their allies to Lepidstatd and tie up a few loose ends along the way, possibly in order to give back the Seasige Effigy which they may be able to recover from Feldgrau.
During this period, you could develop some kind of homebrew plot by either pointing your characters towards another branch of their research, of to show off the WW as a more dire threat that simply a shady cabal responsible for Lorrimor's death. Regarding WW's objective in Illmarsh, just move whatever they are looking for in another location and develop a story which explains why they could not get it until now.

Sunderstone |

Skipping levels 9-10 isn't a big jump if you are using more than 4 PCs and normal experience points. More than 4 PCs typically puts you behind the XP curve anyway. Also if you are using more than a 15 point buy for character generation, it gets easier as well.
If you go this route, you will have to expand Ashes a little more to make up for the XP deficit or Gallowspire will likely end your group, it's a bit of a meat grinder. Maybe an Urban expansion in Caliphas before starting Ashes proper? Or a side quest like running "The Harrowing" module on the way to Caliphas.
If I may ask, why do you want to skip WotW? If you don't like the Cthulu-like flavor, you could omit the whole Migo/Dark tapestry thing (it does read more like a side trek and doesn't contribute all that much to the main story) and turn it into a more Creature from the Black Lagoon theme.
Storywise, maybe Illmarsh is under siege. The Skum now want more than just breeding stock and might have a newer, more aggressive/territorial leader. Keep the Skum, the aquatic tunnels, the devilfish, etc. Add some other cool aquatic foes and you have a chapter a little more fitting thematically. YMMV of course.