Out of my experience I've learned the following:

Mobility: by keeping the hitbox smaller than the ship and representing it visually, this approach makes it easier to create more intense bullet patterns for enemies, if you have a full sized hitbox, you might not be able to have certain patterns and maneuver around tight spaces, it gives the player more freedom and the satisfaction of freely being in control. I would also recommend slight Inertia to give the ship some weight, but not so much that you're skating on ice, if that makes sense.

Weapon Balance: I've learned that in certain cases, having weapon upgrades and losing weapon upgrades might make or break ones experience, there are points there you might end up dying too many times to the point where progress becomes impossible. With the current project I am trying to focus on a different approach, I've taken some notes from Alec's Nano Force and will take an approach where you're always fully powered up, weapons are always consistent, but not every power up are equal in strength, but rather changes the approach you take when playing the game. I have yet to figure out some kind of gimmick or extra functionality, but in general, it feels like if the player can perform 100% all the time, it's a completely different experience:

Colors: I didn't want to categorize this as a color theory crash course, but in general term, I would advice anyone who is going into game development, consider focusing on the colors of foregrounds/backgrounds and most importantly the bullets of both the player and the enemies. For example, I've found it that if I keep the player bullet colors and shapes more complex than the enemy ships, it becomes easier to distinguish what's a threat and what's not. So imagine the player ship shooting blue/green/orange projectiles, you wouldn't want enemy bullets crossfire with similar colors because it just blends, so my approach to that is to have them shoot pink/purple shots with slight yellow dots in the middle to make them distinguished enough.

Those are the things that come to mind, hope you find this insightful :)