I have not read Dumarest at all, but I have read Piper a lot, and his Space Viking, Cosmic Computer, Uller Uprising, and Oomphel in the Sky, all of which can be downloaded from Project Gutenberg, would be very useful.
I am surprised that Andre Norton's Solar Queen series did not make the list, and two of those, Plague Ship and Voodoo Planet, are on Project Gutenberg. Two other ones, Star Guard and Star Rangers, are available as a free download from Baen Books, the publisher.
Star Guard is especially interesting as it pits Low Tech forces against Higher Tech forces.
Harry Harrison's Deathworld trilogy actually covers three planets, Pyrrus, Deathworld 1, the planet of The Ethical Engineer is Deathworld 2, and I forget the name of the third planet. While Pyrrus would not be that useful, an entire planet of deadly plants is not my cup of tea, the other two would be very good. The Ethical Engineer appeared originally as a serial novel in Analog magazine, as did the third one.
I would also suggest taking a look at Christopher Anvil's works as well for inspiration. And all of E. R. Burroughs' John Carter of Mars books are available on Project Gutenberg. They still are great reading, and substitute contra-grav for his lifting ray, and you are home free.