The power button seemed to lose it's "spring'ness" (don't feel it click anymore) and it seemed to coincide with difficulty for turning it on.
I replaced the housing but still had some difficulty to turn it on (a spare motherboard had no issue to 'power on' with the bad housing).

After fiddling with connections and electrical cleaner, it did finally boot and I used it without issue for a few months, never turning it off since I was worried it wouldn't boot up again. It has now shutdown after forgetting to charge the battery.

When I connect it to charger or PC, the front LED comes on solid, then fades away and after 10secs it comes on solid again. There is no Nokia logo or red LED. Flasher doesn't recognise it.

The screens, housings and batteries all work fine with the spare motherboard.

Is there anything else I should try? I'm a bit sad to give up on a 64gb motherboard. Thanks for any advice.

Have you tried this method for flat battery as elicits these symptoms and takes quite a while just to be recognised by flasher:
https://talk.maemo.org/showthread.php?t=87709