We were only at MK on Friday for the
MNSSHP, and we didn't arrive until 6 pm. It makes sense that you found crowds low during the day though...I've read that on party days crowds are lower because many people don't want to use a ticket for MK on a day when it closes early. So though I can't speak to crowds during the regular park hours, I did find the actual party to be quite crowded. I knew from the Disboards to expect crowds, but I hadn't been to a MNSSHP in 12 years, so it was an incredibly dramatic change in crowd levels for me.
We too had both printed out and taken screen shots of our FP+ so that part wasn't an issue (and TBH, rides weren't the focus of this trip for us, with the exception of during DAH, which was absolutely an incredible and decidedly uncrowded experience). We more found that when we were trying to pull up a 4th or alter plans we'd made that
MDE was glitchy, which we had never personally experienced in the parks before (though we had at home when we were trying to do pre-trip planning).
I wasn't there on Sunday or Monday (we were there Thursday afternoon through Saturday night basically...left at the crack of dawn on Sunday). But Saturday was crowded at Epcot WS (as would be expected) due to the F&W festival. As several people here had correctly warned me, weekend crowds would be much heavier than weekday ones. Thursday afternoon at EP was much less crowded than Saturday, and then as I mentioned we went to DAH on Thursday night, arriving at about 7 pm. On Friday we did the AP TS event, which was absolutely incredible, and left HS to go back to the room and nap after riding those rides plus TOT and RNRR after rope drop. We took a loooong nap after doing DAH the night before and then getting up at the crack of dawn for the 7 am TS event, and then slowly made our way through WS at EP (stayed at BC) sampling things from the F&W festival booths, took the monorail over to T&TC, where all modes of transport were broken (including the ferry) for about 20 minutes before they finally let us on the resort monorail because the express one was broken (was anyone else there for that???).