If you want to ride southern Nebraska I would recommend US136. 30 years ago I and some friends rode and camped our way back to Boulder from RAGBRAI via US136,NE89 and US34.

2024 I rode US136 again from Keokuk IA to Boulder on my motorbike. Good road all the way through with shoulders anywhere from 1’ to 3’ and most of it little to no traffic. Not as flat as route 30. I would call it gently rolling hills. Some of the towns still have free overnight camping, I camped in Franklin.

Others mentioned US 20, I rode that and NE 12 (Outlaw Trail) on my return trip and both are fine roads with many historical POI’s . Some of the towns on 12 are farther apart (20mi) There is also a railtrail across part of Nebraska, I don’t recall the name.