We live about the same distance away. I started bringing (driving) my kids when the were a little less than 2 years old. I think the shortest trip was three nights. Even a good traveler would find it difficult to make that drive, have one/two days in the world and then turn around and do the drive again.
I totally understand the desire to do it. Sometimes I think about a taking a day to make it a three day weekend but then we'd be driving almost all Friday, park on Saturday, then driving almost all Sunday. Not so fun.
The thing is... even if she would remember it, (and I actually do have vivid memories as young as 14 months, as does my younger daughter), she doesn't know that she's missing anything by
not going unless you tell her she is. September feels like a long time away but it would be far better to save that money to make your September trip more comfortable. At the price of gas, the gas alone will be about $200, let alone park tickets and lodging.
I do think everyone will have a far superior trip in September. If you were to go in a month or so the crowds will probably be quite awful.
I asked my girls (6,11) if they would rather go on short trips more frequently or stay longer when we do go (say 4 day trips every 8 months or 6 day trips once a year. They both said they would rather go less often and stay longer because of the drive down. They are good car riders (we drive 1300 miles each way each summer to visit family) but it's good to have a nice long tushie rest between there and back.
Finally, what if that one day ends up being a bad weather day. That would be too sad.
I do understand wanting to go sooner. I think about it a lot but the days are creeping quicker to our August trip. I do have a June one booked to WDW too but I'm not sure we'll go on that one.
Great that she's loving Mickey!