I considered not posting as I haven't been in any parks since before thanksgiving (been sick or oncall for work) so this information may not apply to the larger crowds this time of year but any other time this has been accurate..
As a local, I don't really know when I'm gonig to be there so I do same day fp+ all the time. I've been surprised how plentiful they've been all summer and fall, especially if your getting them around noon-2pmish. Still some available near 3-5ish but generally those are for times at the end of the evening only so they may not help out depending on where in the park you'll be by that time. I've grabbed 7dmt some days, fea only once, but the rest of the popular rides and most of the non-popular I can generally get. So the system has been improved, but again I don't know if that translates to very crowded days for the same day fp+ guest.
I've also been playing with a possible way to offset the longer lines when you can't get them. 1: once you scan your fp, it's considered used and the next hour is free to get a new one. for the first 3, if it fits in how I'm going around the park I start to modify and move them up in time while I'm waiting for my turn to ride. after the 3, I start to get a fp for the next ride I plan to go on. I added it up once and we got about an hour back in our day doing this for non-popular, short wait lines. We then waited in 2 30 minute lines so at the end of the day, we still saved an hour in line but it was over a handful of short wait rides and not the big 2 we wanted to go on. It can be confusing but time saved is time saved and if we didn't do this, it would have taken an extra hour to ride everything we did and we would probably have skipped a few rides.