is the GCam app noticeably better than a stock camera app? What sorts of things would it do better?

As I mentioned in my first comment: "Sometimes, the picture quality is decent, but often times it's lacking" when it comes to the ROM camera app. How well a camera does, depends on the app (which is supplied by the ROM) and the driver (usually supplied by the manufacturer). The quality can, therefore, vary from device to device. A few years ago I would've definitely said that GCam is a step above ROM camera apps, but in more and more cases, those have become almost comparable in quality. One aspect where GCam still actually makes a difference is long exposure modes (low light and night photography) as well as offering special modes like panorama or photo-sphere.

The last I checked, the Pixels had excellent sensors but had some camera software issues that I believe were eventually resolved. I’m hoping that isn’t an issue if I’m just using a basic OS.

From my experience, older oneplus devices (e.g. oneplus 7/oneplus 7 pro) and pixel devices (pixel 5 and upwards) have excellent community support, so you should be golden.