You can disable Google Photos, but as you’ve learned, you won’t be able to preview the photo you just took in the camera app (i.e., pressing the thumbnail on the circle on the bottom right).
From what I can tell, your 2 options are to complete disable Google Photos but then accept the limitation that you’ll have to use a 3rd party gallery app to view your photos. However, this is an inconvenience and adds friction, as it will be a multi-step process to quit the camera app and go into another gallery app just to view your photos. I personally just disable the Google Photos back up and use Immich in parallel.
The other option is to unfortunately use a different camera app. But, most people will get Google pixels for Google’s software processing in their photos, so this is the less likely option for people, in my opinion.