Superficially, differences to consider are that the Sonder can take a front rack whilst some of the Arkoses can't, and some of the Arkoses have internal cabling. Can't speak to ride quality or whatever between the two.
No mention of being front rack compatible. Those mid fork bosses are for guards only no? Arkose has the same. Also nothing wrong with internal cabling, it’s not steel so corrosion much less of a problem and it’s not a cross or mountain bike that’ll be covered in mud
You’re describing a pinnacle arkose. No point in going 650b anyway when you can fit +42mm tyres 700c