Syncing iPhone 12 Pro's photos to iCloud is unreliable. Why? How to force-sync?

I'm using an iPhone 12 Pro. When I take photos, I expect them to be uploaded to iCloud so I can access them on my desktop PC, which has iCloud open in Chrome.

However, the uploading from my phone is highly unreliable. When opening "Photos" and going to "Library", it will say at the bottom of the screen "Synced with iCloud Just Now", even when there are still dozens of photos on my phone which I took an hour before, that are still not on iCloud.

One might think that having a button on the screen that says "sync now" would be sensible, but apparently that would violate some prime directive at Apple. Nor (apparently) can it be triggered from the iCloud webpage.

I cannot find any way to manually trigger syncing, so I can get my photos onto my desktop. Even going to "Apple iD-->iCloud-->Photos, turning off and then back on the "sync this phone" slider doesn't reliably trigger it. (It's also deeply frustrating that I can't simply plug a USB-C cable between my phone and computer and access them directly and instantly, as can be done on any Android phone.)

This phone is my only Apple device.

I'm running the latest iOS.

I have it set to "Optimize iPhone Storage".

The files in question are most often photos of a few mb each, not 30-minute videos, or whatever.

I have plenty of space on iCloud (I'm using 38/50 gb)

There are no network restrictions.

My internet connection (6mbps) isn't exactly fast, but it shouldn't take literally hours for a few photos to get from my phone to iCloud.

Any advice would be appreciated.