I'd go legal.

Finish your J visa...

Return...file for a K-1.

You could marry while here and file under the Marriage Visa, but in reality, the K-1 Fiance Visa is quicker and easier.

PS: 6 months is a nice time frame, but in reality it usually takes longer. Figure 4-6 for application approval, then all the medical testing, consulate interview, etc., on your end to get the Visa....