Got bored and looked round the garage at the left over bits from the winters builds
Found the odd new alfine 8 and a 21ah battery and a mid drive a suspension seat post
That's not a derailleur it is an alfine chain tensioner , takes the whip out of the chain under power and makes sure the tension is always correct saves the need for chain adjusters also helps front chain wheel alignment and enables a quick swap down to a 36 front if required it is a hand built German alfine specific frame
But you have horizontal drop-outs, so no need for one. Have you had any trouble with the axle bolts slipping? Without axle adjusters/restraints, that's a possibility if you run with high torque.
It has anti turn washers the wheel has never come loose but the chain is never quite right this is my fourth alfine bike over the last 10 years take it from me this solves chain problems the only time you touch the chain is to replace it after thousands of miles other than a drop of oil first saw this trick on a bike from Japan two versions the jockey wheel or this spring loaded one