"The gunslinger pairs well with any weapon, but try to avoid the wrangler as it is not really worth wrangling one of these tiny things."
False. I use this a lot, it works.
Basically, what Polish said. You are definitely covering the basics to the medium of what an engie should be, however, you are missing out on the most important factor: reading the battlefield. Your impenetrable defense could be destroyed in a second with a single class change on the part of the other team. The key to engie is to be able to read the battle and place your buildings in the most annoying place possible.
Other than that, not bad.
Oh, by the way, you aren't realizing the potential of the Pomson. While it does indeed have the least DPS, it doesn't have multiple pellets or a spread. If you hit, all the damage is done in full force. That is what makes the Pomson so OP.