Minthara's romance does not start until ACT II's epilogue and Lae'zel's romance cannot be delayed for that long, as hers starts in ACT I (either by killing notable monsters or at the post-raid party) and once she's ready there is no way to avoid the conversation as she demands an answer.
But... there is no need to be delaying anything because Minthara doesn't care who the player is in a relationship with. Once she is ready to steal you away for herself in ACT II, she'll make sure to claim you regardless of who your current partner is ^^
So if breaking Lae'zel's heart is not an issue, simply enjoy her romance to the fullest until the end of ACT II and when Minthara approaches to claim you simply decide who you wish to be with.
