Switching regulator (TI LM2678) not working on load > 100mA
It's a common trap for beginners and DCDC converters !
Your inductor is not up to the job.

These kind of DCDC converters there is always a LARGE INDUCTOR, it needs to be large so it can handle the current. Well actually it needs to be able to store a large amount of magnetic energy. I did not see such an inductor in your photos !

You small inductor can only store a small amount of magnetic energy and then it saturates, it cannot store more energy. As a result the output voltage of your DCDC converter drops.

If you replace the inductor with a more suitable one, you have a better chance it will work.
Here are some pictures of suitable inductor types: toroid inductor or SMD power inductors.

Also study the datasheet, ideally you want an inductor that can handle at least 2x the current you need at the output.