Hi LittleSparrow,
I think you could go both ways and there are some sound arguments made for investment in lens as well as going FF.
On the one hand, staying with DX and investing in lens nets you a wider scope of focal lengths to explore whilst going FF targets specifically what you want to achieve, which is to shoot wider and still maintain good isolation with shallow dof.
I won't go into the bokeh quality issues but another lens option you could explore would be a Sigma 18-35 f1.8. Unfortunately this won't make the migration should you eventually go FF but its more or less what you want to do and at a lower cost (more than a stop faster than f2.8 zoom options).
Going FF early is more costly initially (although prices are really getting quite good) but may save you dollars in the long run.
Btw I used to own the 35/1.8G DX and it works fine on FX cameras in FX mode. Saves me adding vignetting in post haha. Quite a lot of barrel distortion on that lens though.

Also Ryan Brenizer's name was mentioned above. And if you don't mind a little PP, you could try out the Brenizer method for static, posed subjects. This method effectively is like shooting with a much larger sensor but in small sections and merged together in post.