I actually joined the Explorer owner club while I was waiting for the kit Just couldn't resist the GAS for a used one for a nice price, hehe.
Thanks!
I think for now I should only sand the sides of the neck pocket. As soon as the neck fits in the pocket I can check the angle of the neck and bridge - nut - distance.... right? I have to remind myself how to properly do that, I think there are some good guides around here.

For the bridge posts I will sand them out a bit further. Too bad I don't have any thinner black bridge posts lying around - that would have been an elegant solution aido78.

Unfortunately, a ~ 0.5 cm x 0.5 cm piece of the veneer came off when i removed one of the bridge posts to sand the hole. I might post a picture of that later. I think I will try to just glue it back in place, do you have any tricks/tips for that?