This is a lot of fun! It took me a while to wrap my head around how to play it and play it well, but once I did I was hooked. It's very satisfying when you reach a massive score. I got around 30,000 on an early level. Reached level 12, at which point it felt nearly too sparse to complete a minimal route, but I really like the makeshift feel at the start of a level to afford building.
Given the short time limit, I spend around half building a minimal route so I can afford buying more pipes, and the other half either optimizing the route or switching between sources, but I never felt satisfied with my solution. Part of it might be the controls where I'd really prefer a drag-drop over swap-click, and something like hotkeys for the shop would speed up building a lot! A "continue playing"-button at the end of a level would be very satisfying, giving you the chance to build the optimal solution .
I couldn't find the sandbox mode, but I think the game has a lot of potential given infinite time. There is of course always an optimal solution, but I could imagine each score-goal reached leading to a new "round" where a few adders/multipliers/sources are added or shuffled around, prices and goal increased, requiring you to tweak your existing machine to complete the new goal, and so on.
Some smaller issues include the numbers moving very slowly or getting stuck after backfiring. I almost never used the rotating piece because of this. I assume the optimal placement is a T-shape where the bottom is the output, but since the sources are packed so closely, I was never able to integrate more than a few sources at once. Either case, great work on this!