Sony Xperia XZ Premium not charging or turning on

I have a Sony Xperia XZ Premium for about 4 years, has worked fine up until now. I got a battery low warning the other day and hooked my charger up - original plug & cable - the phone powered off immediately.

I held the power button and the Sony screen appeared as normal, but the device again powered off straight away. If I left the charger connected it would go through this loop repeatedly, and also the charging light was erratic - flickering green, flashing red etc. It was the same whether I connected it through a wall socket or to a computer, the Xperia Companion app wouldn’t detect the phone either.

I tried a new charging cable but that didn’t fix it. After a day or two, the Sony screen no longer appeared when I held the power button or connected the charger - instead I got the usual battery icon that appears when the phone is powered off and charging, except rather than there being a charge % and the icon filling with green, there was no % and just a single red line in the icon.

I took the phone apart and replaced the charging port - no change. I then replaced the battery which didn’t fix it, but rather put the phone back in the same state it was when it started having problems initially - repeated Sony screen boot loop.

Would appreciate any suggestions!