I am looking for suggestion where opensource Automation tools are needed for web application migration testing

I am looking for suggestion where opensource Automation tools are needed for web application migration testing, something where i can compare two websites side by side and can tell the difference, because i need to test 5800 pages

My Company has an internal web page which is supported by 3rd party apps. by the end of February they are withdrawing the support for our internal website. We are planning to migrate to AEM sites ( different third party) , and as an only tester I have been given the responsibility to find an open source tool that can validate our application web pages (i.e. 5800 pages ) which will help us to safely migrate from old supporting third party apps to new Third party app without losing any data

MyQuestion is : how do we validate that the websites or the pages that are being migrated doesnt loose the data ? and is the exact copy datawise with new improved look

Last option is to pay for the automation tools that can support this, currently looking for suggestions which can satisfy the demands in the open source automation application