Thanks for taking the time posting your concern here in the Community, @alex706.

I understand how the option to automatically set up the billable expense as taxable would be beneficial to your business. Finding new ways to adapt to our customers’ needs is how QuickBooks gets even better. 

Your feedback helps our Product Development team decide which feature will be implemented on the next future update. I'll make sure to take note of this and will inform you once we receive product enhancement update about the availability of this option.

In the meantime, you can manually change the billable expense as taxable as a workaround. 

Please know that you're always welcome to post again if you have any other concerns related to QuickBooks. The Community will always here to help you out. Have a good day ahead.