MacBook Not Starting Up

Hi

I have a MacBook Pro 13” Retina 2014 that does not start up correctly. When pressing the power button it comes onto a blank screen with the backlight of the display dim but on. This occurred yesterday after waking up. I always keep it on charge in clamshell mode and it was outputting the same thing to the external display, for further context the battery was in need of service at about 1200 cycles

Since then, I have tried the following:

Resetting the SMC

Resetting the NVRAM/PRAM

I then assumed this is a hardware issue, so opened the laptop up and tried disconnecting the battery and using the power adapter to see if that has an effect. It didn’t make a difference at first, but when I tried a few hours later again, it turned on; it showed a apple logo with a progress bar underneath it, took about 10 minutes to fill and then went back to a blank screen.. (this is where I am confused since I cannot replicate this now)

I am quite sure it needs a new battery but any help/advice from more experienced people would be greatly appreciated.