Whilst there has been much speculation in the technical media as to the anticipated release of a new model, Apple never pre-announce new products. As such, it is perhaps not surprising that Apple Store staff will not have warned you of an imminent new model. Even if they were themselves aware, they would not have been permitted to confirm details.
Trade-in values are always disappointing - with huge disparity between what you will have paid for a new device and its “value” for trade-in. If it is new, you will almost certainly achieve a better price with private sale - but it will still fall well short of what you may have paid.
As for capabilities of the newest iPad Pro, as compared with yours, there are some differences in the Camera (now a dual wide/normal pair) and the new addition of a new Lidar sensor. How important these are to you, only you can judge. In context of raw performance, unless comparing “raw” benchmark figures, you can expect “real life” performance to be broadly similar.
Chasing the “newest” and “fastest” is always an expensive proposition. If you can afford to do so, well, that’s entirely something for you to decide. In reality, there will always be something new on the horizon.
The iPad Pro that you have now will continue to offer excellent utility and performance for several years to come.
I sincerely hope this is helpful in providing some useful information and avenues for personal contemplation.
