Learn to adjust you derailluer this a need skill. Another reason your shifting is hanging up can be it needs lube. In 100 miles and new then almost dead sure it is cable stretch but once the cable is stretched then adjustments if any should be few and very small. One thing I do when I put new cables on is I put the bike in a stand or hang on a hook by the front wheel. Then spend some time shifting and going through all the gears. This the same thing that guitarist do when putting on néw guitar strings. Yes I repair and play jazz guitars.

Get on YouTube to find a video on how to adjust bikes. There is a great one by a British fellow going out a 10 sp dura ace bike. Basically begin in the highest gear big ring front, small cog in the rear. The cable should be a lowest tension and shift up. The limits are probably already set fine. In the highest gear one shift that takes you to the next cog smooth is the goal. Everything should go ok from that point. If you get shifting but then noisy gears adjust the barrel adjusters. More to it but that should get you going, do it yourself