Pixel 4a 5G Dead Motherboard after screen replacement?

My Pixel 4A 5G started showing a black screen. Initially, this could be solved by force restarting (via Power + Vol Up) the phone. This lasted for a couple of days before the screen seemingly completely died.

As the screen had been cracked before, and the phone still responded (alarm still triggered, vibrated, etc) I was fairly happy that the screen alone was the issue.

I replaced the screen, and the battery using iFixit parts - however, on re-assembly, the phone will not boot, either by long pressing the power button, tapping the power button, or Power + Vol Up.

I've checked and re-seated all connections that are removed during the process, and though I've not sealed down the new screen with adhesive, I'm reasonably happy that it's correctly seated.

The phone still appears to draw a charge (activates a power brick when plugged in), but is otherwise totally unresponsive.

I'm wondering the phone was somehow actually powered up when I first removed the damaged screen, and if so, presumably has short circuited or otherwise damaged the mainboard?

Welcoming any thoughts or suggestions!