I didn't think it was too hard, although I didn't try "hard mode" so I can't comment on that. 

On my first couple tries I definitely didn't know what I was doing, partially due to not reading the instructions, but also because the benefits of the upgrades aren't obvious at first. Not saying this is a bad thing, experimenting with different strategies is often part of the appeal of upgrade based games, especially if it's not a long-term character build that will leave you begging for a respec after months of play.

When I first played I thought that the "Map" button would hide the radar (which by the way, didn't show anything on my computer at work... it's a junker that doesn't support opengl past 3.3 so maybe that has something to do with it?). So, I didn't realize there was a secondary asteroid belt. I launched myself as fast as possible thinking that was the way to reach the star...

After that, I devised a new strategy which in my opinion kinda breaks the game: All storage, no thrust. I also got one mineral processor and one life support. After that it was very easy to harvest 3000 fuel, and just float through to the sun with only 4.5 idle fuel consumption. Repair costs were pretty much negligible at this point and you can abuse the fact that the upgrade screen pauses the game. My fuel only went below 2000 when I was making the final shot.

The problem with thruster upgrades is that they actually make the game harder in a way. It's easier to get out of control and crash, and harder to keep in one place for harvesting fuel. Ironically, dodging asteroids and harvesting fuel felt easier with a slower ship. So the reduction of thrusters became an upgrade for me.  I was playing without radar so maybe that would have made high speeds easier to handle, but what would be the point? My slow tank made it to the sun easily. 

It's a cool concept overall, and I think it could be built upon to make a more well-rounded game experience. I'm not sure if you plan on continuing to develop the game or if you just want "future reference" material, but anyway here's some things that I believe would improve the game:

• More Variety of Tools: I get that the theme of the jam was "multiple uses" and the harvesting with your exhaust idea was clever. But, it would be nice to have other possibilities available. Such as, a weapon that destroys asteroids quickly but yields little or no fuel. Or a shielding unit that consumes fuel while activated but allows you to crash into asteroids without being damaged.
• More Interaction between Objects: I'd love to see things like asteroids colliding and destroying each other, big asteroids breaking into smaller ones, and asteroids breaking if your ship hits them hard enough.
• Less Abundant Fuel: Didn't expect that one did ya? What I mean is, it felt a bit too easy to acquire more fuel. Just find a cluster of large brown asteroids and you basically have infinite upgrades available. I think it would be more exciting to have a reason to go looking for rare but valuable objects. Perhaps the regular asteroids could deplete faster, making the gray ones more important. Or, have a "jackpot" asteroid that releases a big amount of fuel but only when destroyed. It might even be nice to have a separate resource to mine for, such as metal. Some upgrades, like storage, could require metal instead of just fuel, giving the player an intermediary objective.
• Better Handling, but More Dangerous Impacts: The ship really slides around, which is realistic, but then the impacts also don't matter much. Once you have a good amount of fuel, repairs were trivial. I'd consider making the ship a lot more maneuverable, but also making crashes deadlier. Stray asteroids could really be a pain if they just clobber you instantly, but I think with easier control, as well as other defenses like lasers, mines, shields, etc, the player would be able to focus more on flying and shooting, and less on counting fuel and smashing the repair button.

Well those are my 2.5 cents for you,  hope its helpful to you and of course take everything with a grain of salt since you can't please everyone! I did enjoy the game fundamentally, the main issue is that in just a few plays I went from floundering clueless to breaking everything. There could be a lot more in the middle... searching for that next upgrade, flying cautiously on very low health, crashing through stuff with a shield... to keep things interesting and challenging.