Does anyone knows how to bring those dates(from SO) to a saved search report?

You will have to create a workbook analytics for that. I dont think saved search gives billing schedule fields level information

agree, saved search does not access to this data

Thank you, that's what I thought. Is there a brief guide on how to find it in Workbook Analytics? Transaction details maybe?

Go to analytics > Data set > Transaction > Add the Transaction line of billing schedule > Estimated date (Add it as a column)