How can I bake Cannelés more evenely with a big silicon pan?
It's the cookie sheet that's the problem as it impedes air circulation to the middle of the form. The hot air circulates around the outside but it can't get to the middle, so the outer ones get over-baked and the inner ones under-baked. Changing the baking time or temperature isn't going to help. 

As for the solution you have 2 choices (other than a fan oven):


Get a supporting pan below that allows air circulation, you can buy mesh sheets instead of cookie sheets
Cut the form into pieces. Air can't circulate well to the center of that large sheet, so cut it in half and place them apart. They seem to be made of silicone, so a box cutter will go through it no problem. If you cut carefully you won't lose any of the forms, or you could cut through the middle of a line of them