All tours of Denali National Park are done by school bus. I wouldn't be concerned about Regent pre or post cruise being of less qualified operator. In fact, I think (I would have to find the reference) that Regent uses Princess land operators in Alaska for their arrangements. However, when we were planning on MUSH, we were going to do a self driving tour to Denali and a few other spots booking in the same lodges and taking same school bus.

Here is the link to the company we were going to use for our own post-cruise tour of Alaska. You will find that there prices are better than Regent (and probably Princess).