[2]
This idea is that the mc is reincarnated as Rudeus Greyrat in TBATE. The mc will have all the abilities of Rudeus Greyrat and also his memories.
For Backstory
The mc can just have a normal family.
OR
The mc can be reborn a third time. He is reborn from our world to Mushoku Tensei then after becoming the strongest he died an reborn into TBATE