Mostly I can just speculate, since I don't have an older iPad to experiment with. But maybe my rambling will trigger some ideas.
Let me make sure I understand where you are.
You connected the iPad to a modernish Mac with a cable.
Got the two devices to trust each other.
Opened the iPad in the Finder
Used the Manage Storage button at the top of the General tab,
Your files were not there.
Nor were they in any of the other tabs.
Or
You are no longer able to get to Manage Storage when connecting your iPad to the Mac.
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In the first case, you could try making an "unencrypted" backup of the iPad and use a backup extraction tool.
Make sure the iPad is set to save Videos to the backup.
I'm not familiar with the various extraction tools, but if iMazing has that feature, I'd go with it. It's got a trustworthy reputation.
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In the second case, I don't have any good ideas. We'll, if you have a Windows computer available, you could load iTunes on it and see if that will connect. Be sure to pay attention to any dialogs so you don't accidentally erase the iPad. I'm not sure that's still at thing, but it was once early on in the iPad saga.
Sorry not to be of more help.