I’m not sure you already had this question but why did Renly choose to marry Margaery to Robert and not Joffrey? Wouldn’t he get the same advantages without the trouble of removing Cersei?

The point was to remove Cersei. 

Cersei was a clear enemy of both Baratheon brothers (and vice versa), as we see from Bran II of AGOT where she talks to Jaime about all her enemies, or from Eddard XIII where Renly urges Ned to launch a coup against her, or from Sansa V of AGOT where Cersei marks down Renly as a traitor well before he declares his candidacy for the Iron Throne.