Did the new USB-Port Module yield any results?
I have a similar issue. Except for a FP2.
A heavier drop (popped some plastic off) is the only recent misuse that I can recall.
I tried several Charger/Cable combination.
I also went trough a manual OS-Install several times.
In between I dismantled the whole phone and cleaned all the contacts.
The behavior stays the same.
No charging. Except for some initial “juice” until the Blue Battery Shows up.
This is the part that bothers me the most.
So, I dug up a cheap USB-Voltage display and observed, that approximately 1.5A is used during the “red led blinking”-phase.
I guess, until enough power is charged to run the “blue-battery” display.
I noticed that the battery got a little bit warm, when I re-installed the OS.
What I came up with, was to drain the battery until it would boot into “red led blinking”.
Then I would hold volume_down and power, and plug in the charger.
This takes about a minute, but this way I can enter the fastboot mode, and there is a 1.5A usage on my power-monitor-dongle, while the phone is getting warm and the LED is blue.
This is the state right now, and I will wait for about 2hours, to get a fully charged battery.
In the previous run, where I had enough juice to fully boot into the wiped OS for a couple of minutes. Everything seemed to be working fine. No inserted SIM/Sdcard, but Sound, Camera, wifi … all good.
But it did not recognize the available Power at the USB port and started charging.
I am really curious what the Issue might be. I had this device for about 6 years now and already changed the port-module and the battery about two years ago. Back then the USB-port was clearly wiggled out and the whole thing was a minor cost.