Yeah that’s the problem right there. In hex that is 0x8008000, but it should be just 0x8000000 (decimal 134217728), meaning somewhere and somehow, there still is that sneaky 0x8000 offset which causes the firmware to not boot correctly since there is no bootloader on it that jumps to 0x8008000. Execution starts at 0x8000000.
A shot in the dark: Try setting it explicitly to 0.
board_build.offset = 0x0
Is the ISR vector address then correct after a rebuild?