Congrat's on your engagement and I think the impromptu trip is an excellent way to celebrate. ( DH proposed to me (impromptu) during a trip to WDW/FL after college so I may be biased) but it was great having that time to ourselves before all that input from family and friends. It let us really enjoy the moment, KWIM?
My thought's on the trip would be to really think about what you want to do in 3 days, if rides, the DP takes up a lot of time. I know it's tempting because it is a good deal when staying in a value resort, but just saying, it is a time sucker.(plus you won't want to waste
MNSSHP time at TS so if doing DP, plan eat a time consuming meal earlier) if you want to enjoy relaxing more and cool themed restaurants, go the DP route.
Going during options 1 and #2 MAY be a bit less crowded than later, simply because of the FD plan which will bring in some folks and the closer to Halloween for the MNSSHP. If you don't do a park during the day of MNSSHP you can beat the heat a bit, enjoy the pool at the resort, have a nice meal and know that while it will still be humid, and hot, at least the party is when it's dark out for the most part.
Also think about what commitments you have at work before/after the whirlwind weekend, maybe f you know you have a lighter week, book then.
No matter what, you will have your future bride by your side and it will be a blast!