OK, thank you for the clarifications. That does paint a (very) different picture.
A sewing needle. It’s small enough to easily maneuver around that blade. I’ve cleaned maybe a dozen USB-C ports by now – mine, my husband’s, my sister’s, a couple of friends’… You can look and see nothing, because the crud is pressed up against the side. But if you scrape it with the needle it comes loose and then can be removed. And voila, the phone charges again.
Does the charger use a USB-C cable?
USB A on the charger(s) end.
At the risk of beating a dead horse, you’d still get much more power from a USB PD C to C fast charger rated for 20W or higher.
I know that doesn’t directly address the question of “why it charges slower over time”, but it is still much much more powerful than the 7W you’re currently getting at 1800 mA and will very likely solve the issue. 7W just isn’t a lot of power for modern phones under load.
USB PD is a totally different system than USB A to C. It isn’t limited to 5V (can go up to 20V, 30V, or even higher). But you NEED a special USB PD charger for that, along with a C to C cable. There cannot be USB A anywhere. They are all called USB but this is a marketing problem; USB PD is not actually backward compatible with USB A chargers or cables.
How about this? Buy and try that Anker charger. If it doesn’t end up solving the problem, I will buy it off you and pay for shipping. I’m pretty confident it will, though.
OK, I will shut up now! 
+1 on this test device.
This is unusual behavior and I wonder if it’s related to the problem. Perhaps the phone battery is wearing out faster than usual because you have it plugged in most of the time?
Me, too. Although i have an old usb-a charger in my car that pumps enough charge that my phone increases its charge while running Google maps and plugged in. So i would expect an old charger to charge your phone, even under a moderate load.
But also
I’m pretty sure this is part of your problem, as well.
I donno, but the battery is a year newer than the phone, having been replaced.
Before a few days ago it was charging under a very heavy load. As in playing a video in a window while web browsing while transcoding videos and batch reformatting large folders of images in the background.
Does the phone get hot?
Not really. CPU only reaches up around 140 F on CPU-Z, IIRC. Right now my CPU is a little below 100 and the battery around 80 (via CPU-Z).