I think it depends on what you mean by "see most everything"...in other words, do you really mean "everything" or do you mean the highlights? There are lots of attractions, shows, character meet and greets, dining options, etc. and I don't think there is any way to see everything or even most everything with only 1.5 days in each park (3 days total). I think you can probably see most of the highlights in 1 to 2 days per park, but you’ll have to make some trade-offs. I would prioritize which attractions, shows, characters, etc. you want to experience the most and start there.
One thing to note is that August 20th is the currently announced end date for MSEP in DL. So if you want to see MSEP, you’ll need to plan on watching it on the 20th. My guess is that it will be pretty busy that evening as guests come into see MSEP one last time (assuming they don’t announce an extension). Over in DL, there is also F! (debut date announced for July 17th) and fireworks. It’s not yet know how easy it will be watch one or more of the evening entertainment shows. DCA has WoC, which is still a time sink (if you want a good view) so you might want to consider splurging on the dessert party.
August 21st is the first day that SoCal and SoCal Select AP holders can return to the park after the summer blockout. I don’t know if that means larger crowds that week or not (my guess is mostly likely in the evenings after folks get off work or home from school). So while the mornings may be nice, expect some uptick in crowds in the evening (especially if they’re running F! nightly, which will be new and not have debut until after the SoCal and SoCal Select AP blockouts began so that will be their first chance to see the new show).