In order to be the most dangerous man for the marines like he stated, he need to accomplish one of 3 things:
1. Find the truth of the world before anybody else does. That’s basically Robin’s and Law’s goals.
2. Learn about the secrets of Vegapunk. Vegapunk is important for the Marines and WG in general so someone learning about his hidden secrets (let’s say origin of devil fruits, the truth of the world, the meaning of One Piece, his vast knowledge, etc) would prove a huge deal.
3. Finding All Blue. Most people already guessed that you have to destroys Red Line in order for all the seas to be connected and it’s not like you found a good spot and proceeds to destroys Red Line and then the All Blue pops up. No that’s not how it works, just like Raftel, nobody knows the precise location of it, that’s why it’s so hard to find the One Piece unless you have someone with good connection to the Road Poneglyph and Log Pose. If Sanji were to make a mistake and carelessly hit the wrong spot, he could potentially destroys the whole world. If he hit the correct spot, the Red Line will collapse and the seas will jointed as one and the All Blue will appear. Either way, it’s a W for Sanji.
So bar the three options that I listed above, there’s no other way for Sanji to reach the highest bounty in OP unless he beats Imu which is not gonna happen.