Help with Google Apps Script - need to build a dynamic dependent drop down list with minimum 5 dependencies.

I need to build a dependent drop down list using Googlesheets and Google Script Editor. I have found a tutorial that is almost ideal but it only has 2 dependent drop downs and data validation exceeds the maximum 500 rows so I need to use the list from a range criteria instead. This eventually needs to be embedded into our company intranet.
Primary dropdown: Region
First dependent Dropdown: Sub Region
Third Dependent Dropdown: Product Category
Fourth Dependent Dropdown: Product Type
Filtered Result: Supplier
We used to use Awesome Table but the cost is now prohibitive since they introduced pricing based on use.
I can provide this sheet and the script I have started to try to mimic the Awesome Table filtering.
If anyone thinks they can help with this, please let me know how long you think the job will be and your price
I will also need comments in the script to identify what is being done step by step so I can fix/adjust in the future.
I have attached a mock up of what I am trying to achieve if the rows for subregion (column B) were less than 500 (Unfortunately they are close to 40,000) and the script so far.
Thank you
Sandra