The problems are not only with linier and boring gameplay, but also with first 3 upgrades: they could easily be just a 1 upgrade, because they does literally nothing if all 3 of them are not in same tier/level: fuel burn rate in upgrade tier X is exactly equal to fuel generation X which is also equal to fuel tank X, and all of them has the same upgrade cost scaling, so you will always linearly upgrade all them in the row