Yes Slinky Dog for some bizarre reason (let’s face it, it is far from amazing) is difficult to get and you will not get it early in your trip (at least earlier in the day). So if you want it, you need to plan in a trip to HWS later in your stay (and the longer the stay the better), then when the FP window opens, do that first.
I just did mine for Xmas and NY, and this year found the system much faster, I’ve had it before where it wouldn’t even function the servers could not cope. This year was smooth. It’s still much clunkier than it needs to be though (every time you book another pass, having to go back to the select guest screen, no indication of date when you are on the screen to book the pass (leads to confusion), calendar goes back to current date when selecting new date, leading to Fastpasses mistakenly been booked in October rather than December etc argh),
Flights of Passage is easier than it used to be.