Cool start! I really liked how tough your ship was and the emphasis on long term strategy. Repairing your shields was insanely expensive and might be something you want to lower the cost on as it will help newbies survive much longer. I really liked how everything got damaged over time and you really had to allocate funds to repair it. Reminds me of the old classic arcade game Raptor: Call of the Shadows, which if you have never played you should go play right now for an hour or two.
I dug the floaty physics movement and thought you could play that up even more. I also liked the slow speed of the rail gun (more like a torpedo really, rail guns are usually insanely fast, maybe a new feature?) however the length of the cool down really reduced its usefulness.
One suggestion around the pausing feature is rather then actually pausing the game, just give people a break (10 seconds, 20 seconds) between waves to build the ship or vary the intensity. This way you are still putting passive time pressure on them but are able to remove both a full button and an UI from the interface. I also found after 5 minutes it was getting exhausting and could use a break. Just a thought.
Also, you could consider asteroids coming in from different angles as well. This would let you greatly reduce the speed required (for the asteroids) and the total number as they could come in from different angles and that would be much harder to avoid. Be careful with this though, it could easily become over difficult. That would reinforce the strategic elements you already have in place.
I also agree with some of the others that it was hard to tell scrap from the asteroids, although that could be a feature in some ways. You might want to have some more obvious scrap and then some which looks like asteroids but is higher value. You might also want to consider high risk, high reward trade-offs such as an asteroid that you could destroy with a rail gun to gain a big boost of metal . This would give players as skill shot to work on and would greatly increase the depth. It sounds like you are already working on variable difficulty too.
Other people have mentioned sfx, vfx etc and I think those would also really help.
I can see you are managing many uploads. You may want to consider Itch.io for hosting (also allows for long term updates) and especially Butler, which is a command line uploader for Itch that is very cool. Google it and you will find the docs. That is all you need.
Good job! Keep at it!