Uploading photos from iPhone to iCloud Photos, using iOS 14 - is it working / what is the status?
On your iPhone, in the Photos app, at the bottom, select the Library tab. Then select All Photos.
Then swipe upwards.  This should reveal some hidden information about the status of the iCloud Photos upload, which I've highlighted with a red box in this screenshot:

Here, this uncovers that the iPhone has paused the upload, to save battery power. I'm not sure if it warns you about this - perhaps it doesn't, or do you eg have a kid that might have dismissed an alert without telling you?
If you press the blue Resume &quot;button&quot; / link, it'll continue uploading.
The iCloud Photos support document does have a hint about this in the (literally) small print. Emphasis is mine:

Your collection is uploaded to iCloud each time your device connects to Wi-Fi and your battery is charged. When you have iOS 11 or later, your library can be updated over mobile data also.

If you've spent a long time trying to figure this out before finding this information, don't blame yourself. In my opinion, this is an example of poor design from Apple - this feature has low discoverability.