Unless the tool has some sort of “learning” capability that can compare past performance for the rider over various types of terrain as well as elevation gain & loss, I can’t imagine it having a level of accuracy that would make it worthwhile.

That said, those some of inputs exist in the mapping data. If you look at Komoot, they provide data on terrain surface and elevation gain as well as a slider bar to input rider fitness level & they do estimate a time required to cover the course specified. As a matter of fact, Komoot has a process for adding interim waypoints.....it could be that your tool is already developed, just not really promoted for this use case.

There is probably less performance variability with road tourers vs bikepackers so you may want to begin your efforts & or beta testing with that subset of users in mind. They hang out over at r/bicycletouring.