Another thing you might want to consider is the repairability. If you want to swap the battery years down the line, this can become a really important metric as well. iFixit is your friend here: you can just look up a device and check what the repairability score is.

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.

is the Gcam app noticeably better than a stock camera app?

Yes, there's a very noticable difference.

What sorts of things would it do better?

It uses the custom ML chip in the Google Tensor processor for post processing. This makes the photos and videos look amazing.

Low lighting or blur reduction?

Both, and a lot more.

In my other comment, I outlined a solution for easily installing the Google Camera app.