A few thoughts:
- You've mentioned this being Stardew Valley-esque. A big part of that game is the day cycle, which draws you in and gets you in a rhythm. The lack of that is what could potentially hold this game back. If you had to water your plants every 4 minutes it would become an annoyance rather than a part of your daily chores. Same goes for cleaning the air vents or coolant. If that occurred once a day or something like that it wouldn't be a constant hassle, but a part of your daily routine that becomes the rhythm of the game.
- Some sort of notification if you miss a delivery.
- More than just one planet per system to sell random junk. If its a civilized planet you should be able to sell the random stuff you get on uncivilized planets.
- A better way to control the ship while flying would be helpful. Tabbing to "shift left" or "shift right" then pressing the button is adding unnecessary steps when we have WASD controls.
- I never had enough stamina. Sleeping recharges stamina too slowly, and the ship breaks down half way through sleeping either way. After awhile I stopped sleeping altogether as there was no penalty and it wasn't worth it. As you know sleeping stardew valley sleeping advances to the next day and recharges stamina to 100%, I think adding that in this game would solve a lot of problems.
- How long is a time unit? Having 40 units to get to a random planet means nothing, going with my first point a day cycle would help this.
- After getting the warp drive my game froze and crashed when placing an amethyst next to the coolant. Or I think it was the amethyst, it was the item that was "butter colored" in the flavor text.
Best of luck going forward.