Here is the full list of all 53 Persona 5 Royal achievements. It takes around 80-100 hours to unlock all of the achievements on Windows.
Went to a hangout spot with someone.
Obtained a Skill Accessory from a Palace ruler.
Worked a part-time job.
Completed a Mementos request.
Defeated enemies via Disaster Shadow explosion.
Established your own squad of thieves.
Completed the castle Palace.
Scored a hit at the batting cages.
Cleaned up in Leblanc.
Triggered a Technical.
Performed a Guillotine execution.
Used the grappling hook.
Obtained a Persona through negotiation.
Performed a Baton Pass.
Obtained Arsène.
Unlocked all achievements.
Completed the museum Palace.
Completed the bank Palace.
Completed the pyramid Palace.
Completed the spaceport Palace.
Completed the casino Palace.
Completed the cruise ship Palace.
Completed Mementos.
Completed every Palace.
Watched the ending.
Scored the highest on your exams.
Maxed out all social stats.
Entered a special relationship.
Maxed out one of your Confidants.
Evolved a party member's Persona.
Obtained a Treasure Demon.
Performed a Showtime attack.
Traded in flowers while in Mementos.
Traded in stamps while in Mementos.
Completely changed the cognition of Mementos.
Encountered a deviation in Mementos.
Conquered the Reaper.
Performed a Group Guillotine execution.
Used Incense on a Persona in Lockdown.
Performed a Gallows execution.
Performed an execution during a Fusion Alarm.
Performed an Electric Chair execution.
Couldn't take the heat in the bathhouse.
Won the lottery.
Scored a prize in the crane game.
Sold old clothing at Furugi no Neuchi.
Played a game of darts.
Played a game of billiards.
Attended the jazz club.
Visited a temple.
Customized a gun.
Passed time at the fishing pond.
Ordered from the special menu at the maid café.