Here is the full list of all 40 Sonic Frontiers achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Performed Phantom Rush 50 times, outside of the training simulator.
Defeated all guardian types.
Repaired all Portals on Ouranos Island.
Viewed all Side Stories.
Viewed all Side Stories on Ouranos Island.
Raised Ring Level to MAX.
Raised Speed Level to MAX.
Raised Defense Level to MAX.
Raised Power Level to MAX.
Viewed all Side Stories on Chaos Island.
Landed all catches at one fishing spot.
Viewed all Side Stories on Kronos Island.
Completed all Challenges on Kronos Island and fully revealed the map.
Defeated the Titan and completed the main story on Ouranos Island.
Defeated the final threat and completed the main story.
Completed all Challenges on Ouranos Island and fully revealed the map.
Defeated all enemy types.
Defeated the Titan and completed the main story on Chaos Island.
Completed all Challenges on Chaos Island and fully revealed the map.
Repaired all Portals on Chaos Island.
Unlocked all Skills.
Defeated the Titan and completed the main story on Ares Island.
Viewed all Side Stories on Ares Island.
Repaired all Portals on Ares Island.
Completed all Challenges on Ares Island and fully revealed the map.
Earned 100 Purple Coins.
Encountered a starfall for the first time.
Gathered 200 Koco.
Defeated the Titan and completed the main story on Kronos Island.
Repaired all Portals on Kronos Island.
Completed the fishing tutorial.
Destroyed 100 breakable objects.
Spoke with Elder Koco for the first time.
Spoke with Hermit Koco for the first time.
Performed a Phantom Rush for the first time, outside of the training simulator.
Completed the Kronos Island tutorials.
Activated a Power Boost for the first time.
Defeated a guardian for the first time.
Completed all Missions in one Cyber Space area.
Achieved Rank S time, for the first time in a Cyber Space area.