Good strategy. There is a risk that the parks will reach capacity in the afternoon but my impression (I could be wrong) is that they will often re-open in the early evening after some folks start to leave. You may want to ask about being let back in before you leave.
A dessert party is the best bet if you absolutely want to guarantee, but a dining package should be fine and even standby can be fine since you don’t have kids with you. A good standby spot is along the bridge heading to Lamplight Lounge.
Here is a good thread to peruse. A few of us ranked DL rides in comparison to their WDW counterparts. Plus there are some other posts that might be helpful.
Good luck!