I reopened my case since the problem came back. They replaced my iPad Pro. On the first test with the new one, the WiFi dropped 42 minutes into the streaming meeting.
The Apple tech guy in the store was helpful with additional suggestions and an explanation of why this is so hard to investigate - basically, if it’s not a hardware issue, everyone's individual configurations and software make this hard to pinpoint. He suggested that I try to reset my iPad Pro and not restore from backup (which means that I won’t have all of my stuff), and use the basic software to see if it’s truly an iPad Pro issue and not something in my specific configuration/data. I may try this out. I forgot to ask if Apple has tried this test themselves.