GPU issue

I second John. Photoshop now requires at least 1.5 GB of available VRAM in order to utilize GPU features at all. Thus, you will not be able to use any effects that require a GPU at all when you create or export a photo file. And when a GPU was deemed insufficient in any way, the GPU features in Photoshop will be disabled.

By the way, the late 2013 iMac that uses that GPU is now officially in "vintage" support status at Apple itself, and may very soon drop to "obsolete" status.

You need a new GPU card with more memory... and one that is new enough that it is supported