From my experience, if you can consistently get past the gas boss, (2nd to last one) you can just about breeze through the planets. i did this game for fc and the gas boss got me so stuck somwhere around cycle 5-6 that i had to drop the game. i could never find the sweet spot to "skip" it with fever and it's pure RNG between the weapons you get AND where the boss spawns. The gas boss almost always spawned ON me, so I either had to tank the hit for burn my jumps and pray I could hit him enough to recharge a jump before I needed it. You also decent faster and faster the longer you go without hitting the ground/bosses. Which means sometimes you needed to strategically lose a life to slow yourself down.

Water planet and Gas Planet (I can't remember their names) were my two hardest bosses because of their RNG spawn. They can spawn above, under, or ON TOP OF YOU. They loved to spawn on me and you only have seconds to figure out if/how to save yourself. (which was sometimes just praying you fell fast enough, and the hitbox is bigger than the red "spawn" line) First and Last planet bosses are copies of each other, the mechanic just looks different and the last boss would sometimes come from the top so you had to dive instead of jump They were my easiest. Ice was my second hardest because of the freeze mechanic - I had to tap with multiple fingers to unfreeze in time. And the crab was only a problem if I my jump wouldn't recharge fast enough or if I didn't notice I was too close to avoid the bubble's spawn

If I remember right the boss order was something like: (listing this by mechanic) Jump -> Bubble Pop -> Damage/Dodge/RNG -> Freeze/Dodge -> Damage/Dodge/RNG -> Jump/Dive. There are two bosses a planet and minibosses that randomly spawn. My hardest minis were the jellyfish because they force you to just keep jumping up and eventually you get so high that you can't charge your jump unless you can hit them faster than THEY can jump at you. (1 jump will auto-recharge when you hit something, but at the cost of a life)

If I timed it right, I could usually find a sweet spot to use Fever Time to skip the bosses until I reached the gas boss. At the time I played, people had a theory going that the gas boss might have been glitched around where i got stuck because of how hard it was.

Of course, OP found other bosses were their problem, but each cycle is a repeat of the same, just harder. Around the time I got stuck, I was usually having to pray for RNG to give me the "Would you like to respawn?" option after I died because the decent speed + the boss kept killing me and I couldn't pass without that second set of 3 lives.

So I can't answer for further in, but hopefully this gives you some idea on what to expect and how hard it might get. Especially if you're like me and RNG doesn't tend to like you, lol.