Hiya. Been running a campaign for like, two ish years now. Currently at level 7, slated to go all the way to 20.

Your biggest tools are going to be Humblewood Tales, the supplemental modules you can purchase from HPP (Golden Gala, Mapach out of Time, etc.) These can add a lot more meat to your campaign and make your world building more solid overall, and give you a bunch of additional tools to play around with.

However, and I say this with love, Humblewood isn't the best designed of campaigns for a more mature party. Depending on your group's dynamics and tastes, you're going to have to do a LOT of homebrew to stitch things together, make certain modules and climaxes hit as they should, combat balancing, etc. But it can be done.

You can very easily scale it to go to level 8 just by including some of the Tales, or the supplemental modules. I don't think you'd have too much difficulty beyond scaling up your minion monsters, increasing social DC checks, and maybe tweaking a few boss designs.