I'll handle your query about the Tax Liability report in QuickBooks Online (QBO).
The option to show VAT code splits in the Tax Liability report is unavailable. What the report shows is the Net and VAT amount for Standard Purchase and Standard Sales.
There are other options for reports customisation in QBO. Take a look at this article for details: Customise reports in QuickBooks Online. You'll find a list of sample custom reports, along with the steps on how you can generate them in QBO.
Place a comment below if you have other questions. I'll be sure to take care of them.