It really depends on your international destinations...you are well on the way to meeting your goals with DL, but those points are challenging to use. I like AA for RTW, but as others have pointed out, MR/UR don't transfer in.

Have you considered the AmEx Starwood card? I think a lot of folks of FT hold that card, as SPG points transfer 1:1 to over 30 airlines, including DL, AC, and AA. Plus, if you transfer 20,000 points to miles at a single time, you get a bonus 5,000 points (so even though the SPG AmEx earns at 1 point/$, judicious transferring gets you a 25% bonus). With your current spending, that would earn you 50,000 points/year on almost any airline you want, plus no excise tax (IIRC, MR still charges excise tax) on the transfer, and you may find it beats the gas/grocery spend bonus on AmEx PRG.

Still, to meet your goals, you may need to hit a few good signup bonuses as they come around.