If you think you can rout the hole without screwing up your paint job, go ahead. Your paint job will be throughly dry, which is a good thing. The white of the blank will show through the “weep” holes on the flange. Strictly cosmetic. If you use a white box it won’t matter.
Yes I would be a little nervous if I had not done a Futures rout before or lacked enough experience doing them. Waiting to get the kit, doing the rout before paint and then painting is safe.