Good news, I finally completed and published “Spearshield Spaceship”.
As you can see, I made one more major change in the meantime. I noticed that, whenever a fast enemy (such as a rhino beetle) rams into the shield, the player still takes damage. I don’t know, I guess when an object is fast enough, the hitboxes get messed up. I managed to fix the issue by giving the player a thicker shield. It still covers the same area, so the difficulty should stay the same.
As for the saving and loading, I still haven’t figured it out. I’ll probably ask on the Game Maker subreddit. Again, all the levels are available from the start, so it’s not a game-destroying flaw. I’d still recommend you play them in order, however.
You can play the game on my website (just click the “games” section) or on itch.io. Let me know if something isn’t working properly and I’ll see what I can do.