colourful squares over my images

This is a GPU bug (assuming that the file itself is OK when you open it normally). 

Is this a newer Apple silicon machine, or an older one with an AMD GPU? Either way, GPU bugs need to be reported to Apple, but if it's an AMD GPU I don't think you can expect any bug fixes.

As a workaround, try to uncheck "multithreaded compositing" in settings > performance, and under advanced, uncheck "GPU compositing". There should also be an "older GPU mode" checkbox under technology previews.

A new Photoshop version will have some new graphics code, and this can expose latent bugs in the GPU driver.