So you have dispatched below max ramp weight but you have used less than your flt plan taxy fuel taxying out? And you are worried you may be over MTOW? How do you know the accuracy of the ZFW anyway? Performance has margins, ignore the 'excess' taxy fuel and go, provided any exceedance is considered minor. (According to an Ops Manual near me).
It is NOT a requirement to burn all your taxi fuel before getting airborne, therefore you will be within your calculated performance criteria.