What type of transformations do you need to do? Tableau can do a limited amount with iterating over flat files at the moment, so depending on that you might well be able to do everything you need in Tableau.
If it is too complex then I would create a SQL database (if the data is small enough you might get away with Express edition) to host the data. Then use SSIS to feed the data from the flat files (using a for loop to go through each file) and perform the transformations that you need. This will then create the clean data set you are after.
You could perform both data preparation steps and visualisation in Tableau. For example, you can bring files from multiple data sources via joins and unions, you can split concatenated fields and even pivot data. You can also refresh the data (make Tableau look at new files in your folder) without having to load it. Could you describe the transformation process you take?
As for connecting straight to Power BI to Tableau it might not be possible but I found a blog post where the users connect to Power Pivot https://powerpivotpro.com/2014/11/power-bi-and-tableau-best-friends-forever/
Is this what you are trying to attempt?
Why don't you use any database ETL tool to prepare your data instead of doing it in Power BI? It'll be easy to manage then its up to you use Power BI or Tableau for any visualization out of that.