I’ll probably have some more specific guidance with some more details, but I would have a look at the [[MacBook Black Screen|MacBook Black Screen troubleshooting page|new_window=true]] for some ideas. There’s a triage section that might be especially helpful to get you in the right vicinity. I have some theories.
There are a lot of factors here to consider so it might be a bit of a ride. But sometimes these sort of issues are some silly thing.
Also, you mention you replaced the top case. Did the new top case already have a trackpad installed? Or did you transfer it from the original?