If someone has a secondary syphilis rash on their hands and you receive a handjob and only that from them are you at risk of contracting syphilis?

Zero chance of syphilis from a handjob. Secondary syphilis can indeed cause skin rash that can involve the palms of the hands and the soles of the feet (although it is rarely at only those sites -- usually all over the body). Secondary syphilis begins up to a year after catching syphilis, so if you have been at risk in the past 12 months, maybe syphilis is the problem. But not from the handjob.

Unless the rash had weeping sores (not the usual pattern), there would be no germ transfer.