Here is a quick tier list based on the Japanese server.
Rerolling in this game is easy. All you have to do is go to the Login Page, tap on Data Management and delete your game data.
However, since you can keep retrying on your first 10x summon, there isn’t really a need to reroll.
You should aim to summon at least 2 S rank Monsters with your first 10x Summon.
Note: It is possible to get 3 S Ranks
List based on Japanese wiki.