How can I thin a sauce that has too much tapioca flour
There are a couple of things you could try to partially recover the sauce, but my feeling is that none will have very satisfactory results.

The reason is that starch gelatinization is an irreversible process. The problem is even worst for tapioca, which is rich in amylopectin  in comparison with other starches like potato and corn, richer in amylose. Amylopectin is the responsible for the characteristic 'gluey' texture and translucent colour of gelified tapioca (and also asian sticky rice) starch, and is the driving agent of the gelatinization process. Starches which are richer in amylopectin form stronger gels. There are a couple of factors that could have helped slowying gelatinization, like for example adding acid to the water. But the issue is that once the starch has been heated for enough time and the gel formed, acid won't help you go back. 

There are some potential fancy solutions like using amylase to digest the gel into shorter sugars. Amylase is present in your saliva, but you really don't want to spit in your sauce because the other stuff in your saliva will likely lead to fermentation and spoilage of the sauce.

Another potential solution but laborious solution would be trying to dehydrate your mixture and rehydrate it avoiding heating. For example you could bake it in the oven under low heat until the mix gets dry and hard. Then you process it into a blender with water avoiding to overheat it. In Brazil they use tapioca gum to make pancakes, and by personal experience cooking these for two long make it crumbly again.  But I cannot guarantee you this is going to lead you to satisfactory results.