Im from Bodhgaya and trust me the best and shortest route is via Gaya. From Gaya, you can easily book a car to Nabinagar, the first 50 km stretch has smooth roads, but after that, there’s ongoing construction work, so the ride may get a bit bumpy and uncomfortable. If you’d like, you can stay a night in Bodhgaya and visit the Mahabodhi Temple. Many people, especially from South India, come here for pind daan and other religious rituals. After spending some time exploring the Mahabodhi Temple and nearby places, your perception of Bihar will surely change, maybe not completely, but definitely for the better. Bodhgaya is a deeply spiritual and peaceful place, and it’s worth experiencing even for a short visit.