Charge symbol does display, but won't charge or attempt to boot

I stopped using my Galaxy Note 5 about a year ago when it simply stopped charging. I couldn’t be bothered investigating at the time, but I’ve decided to try to repair it.

Symptoms:

1) Charging symbol appears when plugged in but the charging symbol doesn’t fill and the device will not turn on or even seem to attempt to boot while in this state.

2) Holding key combinations for various resets doesn’t turn on the device, but the charging symbol disappears for a few seconds while some of those combinations are held. The symbol reappears a few seconds after letting the buttons go

3) I’ve tried with and without the stylus in the device (not sure if that would change anything)

I’ve ordered a new battery and will have a go at replacing that, but I’ve read many users have had similar issues related to a firmware update so I’m not sure if I’ll have success.

I will update this when I replace the battery.