Is there any way to increase the flash speed for flashing a target from TouchGFX? I have the STM32F769I-Disco board.

Not sure what you really mean ... Programming the internal flash in the F769 and the external QSPI? That's mainly governed by the MX25L51245:

mass erase 140 s and page (256 bytes) programming 0.25 ms typical. This gives approx. 205 s as lower bound for full 64 MBytes.

However, you won't ever come remotely close to this figure as the integrated ST-Link V2 supports at most 4 MHz SWD clock, that's the bottleneck. With ST-Link V3 you might come down to the theoretical lower bound, but: In contrast to the Nucleo boards it's a bit difficult to disconnect the integrated ST-Link and connect another one externally ...

If you haven't already, remove the verify step.

NOR Flash is a bit on the slow side erasing/writing.

Brighter bulbs may have facilitated the use of SDRAM or microSD card as a transitory debug memory during development cycles.