as an fellow developer i also have no idea how to grab user name from itch, there might be an api for that, but i never looked at it.
i hope people arent cheating because some scores seem... impossible.
i remember that back in the days i used multimedia fusion i stored replays by grabing player X,Y position and direction (0 to 359) in an array, but i think grabing replay data might be a better idea.
also spliting the time records leaderboard between legacy build of the game an new builds in case something change or you think people are cheating.
(if the game change enough, it may make 0 sense to keep the record data unless the players can still play an legacy version of the game)