We'll be heading here on a Tuesday next month. Hesitant to book in advance since we're collecting a rental car in Paris beforehand. I'd say we could buy a ticket for the 10:30 slot but I still worry we might miss it. Is it necessary to book on a weekday for April. (We're en route to Honfleur so we'd see it in the a.m. and then move on from there.)
I believe they no longer sell tickets on site; you have to buy on line. You might be able to do that from the parking lot (I don't know if tickets are available last minute) but pretty sure you have to have on line tickets.
Picking up a rental car on a weekday morning at a city office should not be too time-consuming, and traffic leading out of Paris is OK as long as you've picked the right car rental location. I would go for the 10:30 slot, aiming for car pick-up at 8:30. If the office only opens at 9:00, book for 11:00.