The TBB is one of the slowest techniques in the manga to date and leaves plenty of time for the S/T barrier. Any good ninja with decent speed would take advantage of minato's need to stay still while using it, and techniques like amaterasu, susano'o arrows and kamui are incredibly useful against it. Also there is 0 proof that minato can teleport the technique anywhere at will, so until proven differently it can be assumed that it operates in the same fashion as FTG, requiring seals.
Secondly, until shown, minato has no genjutsu feats, even if he did, any genjutsu that imitates the environment such as itachi's ephemeral or sharingan genjutsus are a perfect counter for minato, as they are undetectable unless seen with a sharingan, byakugan, rinnegan, or advanced sensing abilities. This leaves him wide open to attack. As to your claim that he must know how because he's hokage, I would bet anything that Naruto will become hokage by the end of the series, without learning how to dispel genjutsu properly, so this claim is 50/50.