Don't worry.
Particularly when you have 8 days of park hoppers, it becomes very easy to plan around crowds. You'll have no trouble getting fast passes for the later days of the trip. With early rope drops and late nights, plus EMH (if you are onsite) you'll find plenty of opportunities to do the rides in addition to your reserved fastpasses.
We have 8 day parkhoppers for a period that will likely be even more crowded... end of June through July 4th, I've been watching lines carefully in anticipation.
Yes, we will get FOP fastpass for later in the trip. But also plan on riding it 1 morning by getting to rope drop about an hour before park officially opens.
Lines for Slinky Dog and Millennium Falcon aren't so bad late at night, you can hop into a line as park closes.
With 8 solid days, there will be plenty of time to fast pass every headliner, and plenty of time to ride without fast passes as well.
Yes, there is a danger to putting those headliners off until the end -- You can show up with your fastpass in hand, and find the ride is down. We never got to do Splash Mountain on our first family trip (pre-FP+) for this very reason. But like I said, spread over 8 days, you'll find plenty of opportunities to do every big attraction.
And the great thing about 8 day parkhoppers.. you can really pace yourself. You can take long breaks. You don't need to try to squeeze everything in to 1 days. You will be able to hit each park multiple times. (For me, it's 1 morning and 1 evening at AK. 2 full days + 1 morning + 1 evening at DHS, 1 morning + 1 evening at MK, and about 1 morning + 2 evenings at Epcot. I'm confident I'll squeeze in everything I want, except there is a possibility I won't get Mine Train).