Thanks for the inputs! and sorry for being late about this. But I did it with no issues, though it took longer than I expected (a couple of hours) from panicking on bolting things into place and thinking if I forgot anything that went before it.
I only replaced the amount that came out, and figured out that the Differential and the Transmission shared the same fluid. When I replaced the filter and plugged the transmission drain, I left the differential one loose and poured new fluid into the dipstick and eventually the differential drain was leaking telling me it was also being filled.
And from replacing the filter and taking off the “drain pan?” from the transmission, a aluminum rod fell out (about 1x2cm), seems to have broken off of somewhere but from the exposed transmission…I couldn’t tell where it came off from. The drain plug was covered in thick sludge (metal particles and oil) so it makes me think that this wasn’t touched and it was still the original fluid. It had the original cork gasket too. So Now I’m wondering if that broken aluminum piece was critical… When driving, I feel like the van needs to rev too much for it to shift. I need to be 2k and above for it to move like traffic. I thought that normal would be between 1.5k and 2k (since other vehicles do). It also seems to stay on a gear for too long, as in it wants to shift but it eventually does..