Hi Willem,
welcome to the forum and nice looking kit. I recently built an EXA-1. Looks like you need to slightly resize the pup cavities to fit the ESP pups. Or file off the ears on the mount that the pup screws into. Once you have the neck pup snug in the cavity and neck in place maybe if you can please post a photo so we can see if there is any gaps between the neck heel and pocket.
I'll recommend you do another mock build, from memory I had to cut some of the fingerboard below the 22nd fret so I could get the intonation correct. I think you will find the 12th fret to bridge saddle distance too far, so once you do a mock build with pups snug please post the nut to 12th fret and 12th fret to heavy E string saddle distances.
If you are planning solid paint then timbermate colour shouldn't matter but its worth grain filling. I still don't know why people want to put solid paint over sweet ash grain but that's your choice.
Steel wool between coats is more suited to stains, I'd recommend wet sanding between coats and work through to 1200+ grit as it gets closer to final coats.