score:3
You are visiting mid-week, outside of a holiday time, and in early spring. Based on my experience(*), I imagine you would have no problem booking a slot a 1-3 days out, unless you have a large party. Looking on the museum's website right now (as if I was just going to buy a ticket - not using the I Amsterdam card), there seems to be plenty of slots available for tomorrow afternoon. Randomly clicking various other days, there generally seems to be more availability before 10am and after 4pm, so you might want to organise your plans around that.
(*) I used to live in Amsterdam a few years ago, back when the Van Gogh museum was in its temporary home, while the museum building was being refurbished. When I had visitors, we had no problems visiting the museum, just buying tickets on the day. However, things might have changed now the museum is in its new home.
Upvote:1
I live in Amsterdam an work in a hotel. Van Gogh is pretty busy but i think you will have no problem to find a spot. You are right saying tourist season opens and it gets more busy. Specially Keukenhof also opens soon. The i am Amsterdam card is nice but also has this problems. In my opinion its a little overrated. I never advise my guest to buy this card. Just reserve online. You have more options then and avoid this problem. But like i sayed no worries, there is always a time slot left. And if you have really bad luck. Try Rijksmuseum. Its 50 times bigger and why more paintings and art to see. Specially now they are busy with renovating the Nachtwacht and you can watch that live in action ;). A must go if i was you. Have fun in Amsterdam :)