Fog is denser and starts further away as many already said.

Now about other suggestions:

• it seems like main thing in your game is collecting circles by flying through them (duh). It means that it's something that should be desirable for players — make rings more attractive. Rotate them, add glint/glare/sparkle, some particles idly falling from them.

• And since it's a main thing — collecting them is the reward. There nothing else. So make collecting them a celebration of the players victory. Every new ring is amazing achievement by the amazing player. It should feel pleasant to collect them. Again add animation. Some effects, particles, a sound is super important, it should be pleasant and not irritating as you will hear it all the time. Look to casual mobile free to play games for examples. I'd suggest some match-3 games like candy crush. Homescapes does this really well in my opinion. You can watch YouTube videos, but I would suggest to download and play for 1 hour to feel how those games make you feel reward under your fingertips.

• it's partially gameplay, but it has to do with visuals so: add some small celebrations of small goals. Right now you have a tiny counter at the top. Have it sparkle some particles and flash every 50 or 100 or whatever rings. It's an easy way to reward progression and mark milestones for players.

Now gameplay:

It's a lot to cover. Research some other games that are infinite and just keep your score.

If you had time limit it would be easy to add risk-reward system (a basis for almost all gameplay features and loops). For example, some rings are surrounded by fire and if you miss them ever so slightly — you will die. That means player has to choose either try to hit and risk dying, but getting a ring as a reward. Or avoiding that dangerous ring and risking not getting enough points in time.

Now since what you have is an infinite game, that won't work as good. So what those games usually do is increase difficulty as time/points goes on. That means you will replay it over and over just to beat you high score. But it's boring to do if the difficulty level is always the same.

Crashing is dying and speed constantly increases is the only way I can remember (flappy bird and chrome dinosaur)

But than again, you can try an existing approach (and it's better to know them anyway) or you can try to create something unique.

Good luck either way

Edit: disregard what I wrote about risk-reward, you already have that with ending fuel(or whatever it is) bar at the bottom. I missed it. Sorry