To be fair, no one knows what the crowds will be like with Island Tower because we haven't crossed that bridge yet. There are educated guesses at what a total capacity figure might look like, but we don't really know how the crowds are going to behave (for instance, if you are staying at Island Tower and going to Magic Kingdom, is it better to walk to the boat launch and get on a boat, or walk over to the Great Ceremonial Hall (lobby), and go up the stairs/elevator to the second floor, to get on a monorail that's already passed through Contemporary and the TTC?)
I can, though, tell you what crowds were like at Grand Floridian this past summer.
The Magic Kingdom Monorail: At no point were we unable to load onto the next monorail to arrive. Was it packed? Sure was during park opening hours. But, it's packed for the like two minutes between the Grand Floridian and Magic Kingdom -- hardly noteworthy unless someone forgot to brush their teeth or wear their deodorant. Going the other way, I'd opt to take the boat from the Magic Kingdom to the Grand Floridian unless the monorail line appears to be light or a boat JUST left.
As an aside, I'm still holding out hope that when Island Tower opens, WDW will restrict the ability to get on the resort loop from the TTC -- Turn the TTC into an exit-only for the clockwise monorail. At least during Magic Kingdom open. That would be very simple and would only require them to invest in some signage and gates. I haven't heard any plans to do this, but gosh would it make sense.
The EPCOT Monorail: Different story. We loathe this thing. Except during park open/close, we have resigned to taking an Uber to EPCOT from now on if we are staying at any of the monorail resorts. It's not so much the crowds as it is the inconvenience along with the crowds. Nevertheless, I don't recall people being left at the TTC. I'm sure it happens on occasion, it just didn't while we were there. During park open/close, Uber can be tricky because you have to deal with the parking traffic -- often the EPCOT monorail is the more efficient mode at that time.
The Buses: Usually during peak times (open and close) in the summer, and during other times of the year, they'll run separate busses between each resort and the parks. They know that these are high demand times. They know you're going to be upset if you can't get on a bus in time for park open. And, further, if the crowds get out-of-whack and the busses somehow become a problem, it's pretty easy for them to adjust bus routes to add in extra capacity. I recall a time when we were staying at Beach Club and the
ECV line for the Magic Kingdom was getting long - the driver called for an additional bus and it showed up within 10 minutes. I wouldn't worry too much about the busses unless your trip was the week before or the two weeks after Christmas (when they might be ironing out some of these crowd flow issues in the absolute THICK of it lol). By summer, they'll have the busses humming along efficiently.
@Mle8308 is right in that they do combine stops during off-peak times -- how that impacts you is really a crap shoot as there's no way to predict whether one or 20 Polynesian/Grand Floridian guests will be wanting to leave Hollywood Studios at 2:00 on June 15, 2025. But this is true of all of the Deluxe resorts except one -- it's really only Riviera that is always non-stop between the one resort stop and the parks (the rest either have multiple internal stops or share a bus between two or more resorts).
For what it's worth, this is one of the bigger reasons why we prefer the Riviera to the Grand Floridian. We'll take the Skyliner and a dedicated bus over the monorail 10 times out of 10. But, I recognize that's not really what you asked...