I am just giving out levels / tiers at the chapter breaks I listed above. AKA 'you finished this chapter, you gain level 12'. There have been one or two times if they did something especially impressive I gave the boost early, but for the most part I've stayed on track.
Honestly, it's been hard for me to keep them waiting, too; but it's hard enough to balance the campaigns as is with a group of 5 min/maxers, so rushing to make them even more OP would only end up hurting myself lol.
As a power gamer myself, I'd probably be bothered by the slower pace, but they don't seem to mind; if it became an issue, I'd just doll out xp points to create the illusion of progress, like taking surface streets to avoid highway traffic. It may not be any faster, but at least it feels like you're gaining.
*Note, I'm not going to rearrange things to finish Rune Lords before Midnight Isles after all, since it will make balancing encounters too big of a headache, so tonight they head in to close the Midnight Fane!
I will say this though, running these two combined story has been lengthy, but well worth it.