I want to visit the Danxia landform next year. What options are available for tours from Zhangye?

I highly recommended to travel on your own to that place. Or you can join a local tour to visit there.
There is no physical efforts required to visit there.
Taking local public buses: From ZY Railway Station, you can take a taxi to reach local Xiguan Bus Station where you can find buses for Sunan. Then tell the conductor you will reach Danxia and they will remind you to get off when the bus reach there.
In my opinion, hiring a local car is suggested to travel to Danxia. The taxi fare is around 120 to 150RMB.