migrating / merging iphone contacts I have an iPhone 6 (with proper iTunes backup) where all of my contacts where, but I bought a new iPhone 7 and had to initialize it as new iPhone, due to several time-related constraints, so now I have a few but new contacts on my new phone, but none of the older ones, but would be the best practice oriented, for just recovering the old contacts from the iTunes backups?

PS: I still have the old iPhone 6, so it could be used as "temporary storage" if needed.
You could restore the old backup to your iPhone 6 and sync those contacts with iCloud, then sync contacts on your new iPhone 7 to get those. (As fsb noted above)
Some third party backup reading tools allow you to export contacts. I'm a developer for Decipher Backup Browser - you can use Decipher Backup Browser to export the contacts from the backup as a VCF file, and then email/text/dropbox/etc that to yourself on your new iPhone to import the contacts. (When you open the attachment on the iPhone 7, one of the built-in VCF actions is to import to contacts.)
You could setup a gmail or similar account to sync contacts temporarily -- basically the same idea as syncing contacts with iCloud, but not iCloud if you're opposed to that.
I would not recommend it, but you could restore the backup to the iPhone 6, and then go contact-by-contact pressing Share to send the contact to your new iPhone.
Do you have a Mac? You may be able to restore to the iPhone 6, sync contacts on the 6 with iTunes, and then share multiple contacts to your iPhone from the macOS address book app? https://www.tekrevue.com/tip/share-contacts-mac/ (disclaimer: I didn't try doing this to get the step-by-step. It's just an idea.)