IMHO arms and feet power are to much in one machine ?
Your calves can take all the oxygen you can process [ biggest muscles in body ] so arms if used are only taking that away from the calves.
The excuse if your legs get tired is false , if your legs are that tired it is time for a break with a overload of caffeine and sugar about 20 minutes usually does it , then fresh for the next 50 miles
I had toyed with the idea of a delta with slightly [ free ] castoring rear wheels a bit like a spin cycle but with 20" wheels all around ?
maybe restrict them to a 20' arc either side of straight ahead ?
Why ?
Well it would stop inaccuracies in the rear frame creating toe in or toe out at the back ?
Should allow a tighter turning circle ? important in an urban environment.
However what little trike time I have could be better spent making rear suspension or a better front end me thinks.
Paul