The combination of tower defence and factory is quite interesting! I believe there is a great potential behind this hybrid genre, expecting you could keep exploring and expanding on it!
For the factory part, I believe some great game of the genre can inspire you a lot; it should not be just about resource producing, but also about detailed pipeline arrangement and planing. There are just so much possible to play in this part.
The output of the factory would be directly used in the tower defence part, which creates a tense core-loop of the game. However, I do find the strategy should be much deeper; now, it's just about building more and more units producer, which is more like incremental but not strategy.
Overall, quite potential a combination! Expecting a polished version of it!