The reason you can’t find a 5 mg scoop is that every single powder has a different density. Even if you find one, you would need a scale anyway to make sure it’s even close to accurate.
Also, as said, there is 100% chance there are fillers in the capsules. You’ll have to account for that.
Literally, I would just eyeball it and get pretty close to half. If you want to be super picky, get a milligram scale and weigh it out to get exactly half. If it’s something you’re taking daily, even if you don’t get a perfect half, you’ll still be averaging 5mg per day, so it’ll be close enough.