Instead of attaching transistors to shift registers there are shift register with open drain outputs and led drivers with shift register like interface designed

Instead of attaching transistors to shift registers there are shift register with open drain outputs and led drivers with shift register like interface designed for usecase like this. Some of them even have constant current drivers builtin which would allow eliminating the resistors further simplifying the build although that might require higher supply voltages for longer LED chains which might not be ideal for this project. Attiny85 operates at the same <20Mhz as ATMega328 in arduino pro mini lack so first one being too slow shouldn't be a thing. From the screenshots it seems that output enable pin wasn't used so I assume fading was done by quickly changing picture. A better approach would be connecting the output enable pin of shift registers to hardware PWM output of microcontroller that way fading can be controlled independently from picture content and easily run frequency where flickering isn't visible even for camera. You wouldn't try to dim a monitor by rapidly changing the picture, same situation with LEDs.