If you look in the Settings menu in your Google Photos app, there's a 'Free up device storage' option that will do what you're asking. But I'd give it some thought though. Is your photo library stored on your Pixel really taking up a significant amount of storage space?
Keep in mind that when you intentionally have your photos deleted from your phone, your photos are essentially not backed up anymore. Your photos will be stored in your online Google account, and no longer on your phone. Google's server farms are very reliable and stable but the issue you should never ignore is if something happens with how you access your Google account (i.e. some a**hole hacks and takes over your account, or you're in a location with spotty or no WiFi nor mobile data/cellular), Or in other words, whenever you can't access your online Google account, you can't access all your photos.