iPod Touch 6th Gen won't boot, won't charge, can't restore (4013)

A mate of mine has given me a pair of 6th gen iPod touches to fix because I'm a computer technician and as a result have some knowledge of apple devices and have the tools if a hardware issue is present. One of them will need a screen replacement but I digress.

Both of them are doing the same thing.

• When unplugged, no response at all to attempting to power them on (they've been out of use for a while so a fully dead battery might explain that).

• When plugged into a 5W+ power source they power on immediately and display the apple logo for about 5 seconds, then the display turns black and stays that way (my 7th gen stays on the logo for about 15 seconds and then displays the lock screen). It doesn't seem to be a power loss, as the backlight is still on, and if I hold the power button for 5 seconds the power off. They automatically power back on as soon as I've let go of the power button. No charge screen is displayed ever.

• When trying to connect to a computer (a high end Windows 10 gaming PC, as I don't have a Mac on hand, and neither does he) nothing happens unless plugged in via a powered USB hub that delivers at least 5W. My 7th gen connects and charges just fine off of a 0.5W USB 2.0 port, but these don't react even on a 4.5W USB 3.2 Gen 2 port. They only power on if 5W or more is delivered.

• When plugged in via a powered hub, they both connect to the computer, but do not appear as a device with browsable storage like my 7th gen does (unsure if this is a limitation of the 6th gens or an older version of iOS, my 7th gen is the first I've owned), but they do react with iTunes. iTunes immediately asks to update or restore them, and attempting to do so (with permission from my mate) results in about half an hour of the progress swapping back and forth between "Waiting for iPod" and "Preparing to restore iPod". Eventually it will result in a 4013 error.

• Several times during the restore process before the error appears, I'll hear the disconnect and reconnect jingle from windows, but the one time I needed to restore my 7th gen this happened as well, so I had been assuming that was normal. However after they failed to restore for about the 5th time, I stopped trying for a while, leaving one plugged in, and kept hearing it disconnect and reconnect every about hour, so something else might be the issue.

I've been searching online for fixes, tried everything listed on the page for the 4013 error, as well as other recommendations on forums etc, even a brand new cable to make sure that wasn't it, but nothing has worked. Also force restart isn't an option because they never boot in the first place, but I tried that anyway just to see and they just react the same as if I'd just held the power button.

I was hoping I could fix this issue without having to open them (aside from the screen replacement), but at this point I'm feeling like it might be a hardware issue, such as the port itself or the battery, Unsure if this assessment is correct, but given they are both doing it, and only one has external damage, the long period of being unused seems to be the only common factor.

Is there anything I can try before ordering replacement parts?