2x MacBook Pro Retina continually crashing after battery replacement

As title suggests, I have two of the same Macbook Pro laptops with the same issue

Macbook Pro Retina mid 2014 - Used this laptop for years until it came time to replace the battery, I replaced it myself and it worked fine for a couple of hours before it randomly started crashing. I got sick of this so bought another used Macbook Pro of the same model. After a couple of years it came time to replace the battery again, I didn't want the same issue so I took it to a computer shop. They replaced the battery, I brought it home, worked fine for a couple of hours and then same thing again, just starts crashing randomly.

What have I tried so far?
I've essentially deleted every app possible from both laptops
I've run the Etrecheck report and followed advice by those on Apple Community forum
I've installed a new OS on one of the laptops (both had OS X Yosemite, now Big Sur)

Does NOT crash while in safe mode or using a guest account

I've tried to find anything that triggers the crash. Often I can watch movies or something on VLC no problem for almost any duration of time, however if I do anything different especially opening a web browser - crash.

If I leave the laptop to go into a screensaver/sleep mode it will crash
If I try to navigate while using a web browser it will crash

At this point I have two Macbook Paperweights, I'm open to anything. Please help me.