I think, Grt-Bt thermometry not good solution for granites. Calibration was found only for metamorphic rock. If used 'classical' methods may be use two feldspar thermometer, Al in hornblende (magmatic!) barometer, Ti in zircon / titanite / rutile or quartz thermometer, Cl-F in apatite-biotite thermometer.
But moo correct in my point - use pseudosections methods (Theriak-Domino, Perplex, Thermocalc). This soft uses thermodynamic database for minerals and solid solution.
Biotite as a petrogenetic discriminator: Chemical insights from igneous, meta-igneous and meta-sedimentary rocks in Iran
DOI: 10.1016/j.lithos.2021.106016
In any magmatic or metamorphic system, mineral-mineral thermometry is applicable if they were crystallized under equilibrium conditions. So, first a careful microscpic investigations is recommended to judge wether garnet is a primary crystallized phase or xenocrysts. In case, the garnet is a primary magmatic phase one should try to test wether it was crystallized in equilibrium with the other mineralogical phase or not.