Offline initial setup from previous phone What's the best way to copy everything (apps and their data, contacts, sms, phone settings if possible...) from a nexus 5 to nexus 5x, both using android 6.x, in offline mode, ie without using any google account or similar ?
There won't be an easy way covering everything, but the best approach most likely is:


using adb backup / adb restore to cover 95% of the stuff (see Full Backup of non-rooted devices for details)
catching the remains (SMS, probably also contacts) with a specialised app (as for contacts, you can also use the contacts app itself: Menu › Export/Import allows to export to/import from SD card)


Alternatively, you could also take a look at Titanium Backup. This would also be a good idea for later "daily use", to always have an up-to-date backup ready just in case. To use it reasonably on a daily base (and unleash its full power), you'd have to buy the license, though – which is a few bucks, but definitely worth it (this one one of the first Android apps I've obtained, and I'm still using it happily). But in this case, be careful with restoring "system stuff" to a different device/ROM (make sure at least you've activated migration mode), or you might mess up things (which I did once when restoring SMS, as the database structure had changed).