We do not have the right to shorten the Sandhyavandanam on our own accord. During the Theetu (period of impurity) and pradosha time, it should be shortened, and at other times, it should not be shortened, is what elders have said. Not having time is not a reason to shorten it. However, if it is just for one day, it is okay. Even before attempting this, you can try starting Sandhyavandanam earlier. You can do it before sunset or sunrise. When travelling, do it early. In the points you mentioned, anganyasam and karanyasam were excluded. I do not have the authority to say that one should omit those and perform them. No one has the authority to shorten the Sandhyavandanam.