How to diagnose insertion point/trackpad jitters?
I'm sure you've read Apple's support documentation on this issue by now and have had no luck.

Since the issue wasn't resolved even after reinstalling macOS from scratch and booting into safe mode, this should definitely be a hardware problem. Your trackpad is slowly dying. The connector is probably loose or getting old. Since you say that the trackpad cable has already been changed once, it probably wasn't serviced properly the first time.

The best thing to do here would be to go back to an Apple service center and get your laptop checked again. That's almost always the best route to take in case of hardware problems with Apple products.

FYI: There are people who've said that a firm slap with the palm of their hand fixed their trackpad issues. Try at your own risk though.