Hey Adrian_NL,
Thanks for using Apple Support Communities! I understand you have some questions about getting your data from your iPhone 6s to your iPhone 6. I definitely understand how important this is, and I'm happy to help.
While restoring a backup done on iOS 13 to a device running iOS 12 isn't an option, you may be able to still get some information on your iPhone 6. iCloud back ups do not contain information already stored in iCloud. That means if you were using iCloud Photos, iCloud Contacts, or storing other information in iCloud it would still be available just by signing in on your iPhone 6 with the same Apple ID.
You can also use the link below to re download any past purchases on your iPhone 6.
Redownload apps, music, movies, TV shows, and books
Thanks again, and have a great day!