I don't think things will be super, super bad. Pixar Pier is just one area of DCA. When GOTG opened last year, the Hollywoodland area of DCA was very crowded. But over in DL, we didn't really notice a massive, oppressive increase in crowds. Get parkhoppers so that if you find the DCA crowds getting to be too much for you, you can easily hop on over to DL for a while. Get MP so that you can book FPs for one park while you are still in the other. Try to eat at off peak times so that the restaurants aren't super crowded. You might want to look into some dining packages for shows (F!, WOC, PTN) so you can have easier viewing. Download the free DLR app ahead of time so that you can be keeping an eye on wait times and FP availability/return times throughout the day during the week or so before your trip. That will give you some idea of what to expect.
I think
@theluckyrabbit is mostly on point here but maybe not totally giving sufficient attention to the SCAPs.
A dynamic that will impact you on Sunday and Monday are the SCAPs aka So Cal AP holders. They are blocked from going to DLR June 20-23. They will be back in droves June 24-28 before the entire summer blockout sets in and they will be focused on DCA and PxP. Your Sunday and Monday will be especially impacted by them. See image below.
Maybe try and do DCA before June 23 when PxP opens when crowds will be more moderate? Then expect a surge starting starting on Saturday June 23. But without SCAPs or DAPs (Deluxe AP holders are blocked on Saturdays) Saturday may not be too bad even though it is opening day. I expect Sunday to be by far the worst. Go to DL that day.
So now I have worked out a plan for you! Parkhoppers are not always needed at DLR but this is a case it will be worth it for anyone in your situation to get parkhoppers.
Thursday/Friday - make sure you take time to see DCA before the surge. Might be a good time to see Paint the Night.
Saturday - Get into DCA to see PxP at some point and expect things to be crowded. Might be a good night to see World of Color.
Sunday - no brainer - stay at DL all day - if you are so inclined see the new Pixar fireworks this evening.
Monday - your call, but PxP will be pretty bad.
We will cross paths with you as we are visiting June 19-23. We were not planning a park day on June 23 and just flying home. But that was before they announced PxP opening. Now we are planning to head into DCA for EMH at 7AM on Sat June 23. We will then go back to the hotel and checkout so will only have a few hours dedicated to PxP. With EMH and MP we should be fine.
ETA: Ooops, no EMH at DCA on June 23! So regular opening which as of now shows on the calendar as 9AM but I have to believe will be changed to 8AM.
Hope that helps!
https://disneyland.disney.go.com/passes/blockout-dates/ see screen shot below.
