This seems like a perfect opportunity for a compliment sandwich.

"Hey DM, we're all having a lot of fun in our weekly session. Do you think you might give us some more opportunities for combat, etc. All the world building is cool, but sometimes we can go a session or two without fighting anything or exploring a new location (etc.).

Thanks again for running doing. We're really enjoying this setting that you've created for us."

The work the DM is putting into this campaign is probably very personal to him/her. So coming right out and telling someone what you don't like would be like telling someone they write bad poetry.

Then, the next time you start a new campaign, suggest a published campaign with good reviews and then make sure to really compliment the parts of it you liked.