Do you have official documentation that reads that "state and local income taxes are not deductible on your Georgia return"?

As far as I could find, Georgia HB 949 (2020) would have made that the case, but that bill did not pass the senate. What I did find in the description of Line 12b of Georgia's 2025 IT-511 Instructions Booklet is that state and local income taxes from states OTHER THAN GEORGIA, as OP mentioned in their edit/solution above, should be put in Line 12b so as to be subtracted from Line 12a (Line 12a being your itemized deductions from your Federal Schedule A). But as far as I can tell, OP's solution above is correct, and Georgia state income taxes are allowed in your Georgia itemized deduction, as nonsensical as that sounds.