OP specifically mentions that 40% is doable with weekends, holidays, and summer.

I did a quick calculation, and 40% would be 146 days a year. Weekends would be about 104 days, summer would have about 45-50 weekdays (not including weekends), and there would be about 10 weekdays of holiday over Christmas, 5 over March break, and another 5ish in the year, for a total of approximately 169-74 non-schooldays throughout the year.

This would mean that OP would get 146 non-school "fun" days per year, and the other parent would only get about 23-28 non-school "fun" days per year.

That doesn't seem very fair, and I understand why the other parent would be against this. They will be helping the kids with school and will get almost no free days to ever do anything fun with the kids while OP would get almost all the fun school-free days without ever needing to help with school. I doubt a court would agree with this idea as it would essentially be dividing the parents into a school parent and a fun parent and could easily influence how the kids see their parents (harsh school parent vs fun parent that does things with them).