Thank you for your detailed Feedback! I am glad, you enjoyed the game.
Tubes are required for some of the higher upgrade levels, as upgrades need additional material types later.
Yes, there is an ending:
MINOR SPOILER AHEAD
Far to the left is a unique boss alien significantly larger and more powerful than the usual aliens.
When beating it, you win the game. Afterwards, you can continue playing, but obviously then there is no real challenge left, currently.