Check out some simple clicker games on mobile. Observe the main game loop, UI design, visual cues, sound feedback. You don't have to copy them with 100% accuracy, just get the basic idea and mechanics, then prototype it. Eventually you will end up with many small prototypes and think of combining some of them enough to make a small game or minimum viable product (MVP).

In your game, your primary loop is probably the space travel because you spend more time in it the more you upgrade the fuel. Mining/clicker mechanic is the secondary loop for obtaining resources. What you would want is to make the primary loop feel more fun, lasts longer, and happening frequently than secondary loop.

I got few ideas to add up to your primary loop.

1. Discover new planet after travelling a certain distance - You can see this in common runner games with distance meter at top screen. Unlock something after reaching a milestone, achievement, new item, new upgrades, etc. In this case, the planet can grant resources and materials.
2. Random events and encounters - As you travel, you might find an abandoned ship with resources, or found a rare asteroid you can mine for rare ores, or found a fellow astro-miner to help you on your next mining. Just a pop-up message with options.

After you finalize the whole idea, you can polish the game then focus on contents.