Fan was overspinning, now my MacBook Pro is unusable

Hi there, I’ve got quite a conundrum for the community - details below.

early 2015 MacBook Pro, Mojave 10.16.4

> woke up the other morning and opened my laptop, it wouldn’t wake as normal (stayed on black screen) so I did a forced shutdown and when I rebooted the fan was spinning out of control

> this has happened once before, so I tried to perform an SMC reset to stop it, this didn’t work so I reset the PRAM. Neither worked but tried both multiple times.

> battery also behaving weirdly - showing as connected to power even when not, is not appearing in top bar, won’t let me click the box to have it appear in the top bar.

> realised I was using the silicon-based SMC reset method (idiot), switched to the Intel one and it works first try. Fan is chill.

> boot the computer, it’s very slow to boot.

> restart, now the computer will not restart when I press the power button like normal. It turns on but power cuts out immediately. There is no chime or apple logo.

> computer DOES turn on when I hold the power button for a few seconds and stays on.

> it is now so slow it’s unusable. Roughly 10 minutes to boot up, multiple seconds thinking about each letter I type etc.

> try many things, smc resets, pram, etc. nothing will make it run faster.

> boot in Restore mode. Repair HD with disk utility. No dice.

> boot in Restore mode. Reinstall OS. No dice. The reinstallation takes about 12 hours which indicates it’s still running at snails pace.

> I’m now doing a backup and then will erase the HD and try to start again.

anyone have any ideas as to WTF happened here or whether I can fix it?