'Include Tax' - this parameter is for product prices.
transactionTotal - order total with tax and shipping fees.
transactionTax - order total tax.
I can add the new parameter 'transactionSubTotal'
transactionTotal - order subtotal value (with or without TAX)
What do you think?
No, because Google Analytics only looks for the variable transactionTotal, it won't recognize transactionSubTotal.
You should make another checkbox in the Settings called 'Include Tax & Shipping In Order Total'. If unchecked, then you don't include tax/shipping amounts in the transactionTotal variable amount. If checked, then you include tax/shipping amounts in the transactionTotal variable amount.
Then you can also rename the existing Include Tax setting to be called 'Include Tax on Products'