It sounds like you only want to build the frontend yourselves? You could take a look at Cube[1] which will probably work fine as a backend for your in-app reporting.
It's the requests for follow-up questions that can make building and maintaining your own version tricky.
fwiw ... you might be "successful" doing what you're proposing (if you choose a tiny feature set, and discard everything else everyone asks you for)
...but the reason "out-of-the-box solutions" exist is because other folks have already spent the millions of developer hours, user feedback, etc to get it working :)
Several years ago I worked for a company (filled with seasoned, smart, great devs) that tried to in-house their own reporting tool
They quit and migrated to Crystal Reports because integrating an existing tool was worlds cheaper/faster than trying to do it all themselves
Later they migrated to BIRT because of CR limitations
And did another migration a couple years after that (I forget now to what tool)
The only drawback from a end-user perspective of those migrations were that reports couldn't be automigrated between reporting platforms