Startup fails after battery replacement? Boots, but doesn't finish. :(

Completed the battery replacement guide yesterday following all the steps. I tried to start up my laptop but no success. It gets to about 2/3 of the progress bar on startup, then crashes and tries to reboot.

I have reset the SMC, NVRAM/PRAM and tried booting in recovery, then internet recovery mode. It fails each time.

This was my first time using iFixit or attempting something like this. Please could someone offer advice on a fix before I admit defeat and take it to an Apple store?