Solved! Not fun, not pretty. It took three times with Apple Support, two by text, one by phone (elevated to Apple Expert, live). Super Expert help, friendly and empathetic. It’s a known issue.
The only solution at this time is to wipe the iPad to new status, down to “hello” screen. When it asks if you wish to restore from backup, skip this step. DO NOT restore from backup, instead, go through setup to restore it to “new.” Pick your language, country, etc. etc until it loads the standard set of apps and welcomes you. Then, wipe it again. Yes, that’s right, you must wipe it twice, back to “Hello.” This time, when it asks if you wish to restore from backup, say yes. After restoring from backup, my iPad system data dropped from 200 GB to 6 GB. It’s stayed that way for over a week now.
It was explained to me that Apple engineering is working on this, but it appears to be system data dumped there by third party apps. Some app on my iPad is dumping large amounts of system data that isn’t actually used, and it builds up. Delete that app completely and it goes away. Problem is, which app? You can delete apps one at a time to figure out which one, or wipe it all to clear it.
I had a strong suspicion it was the app for my outdoor hi-res video cam(you know which one - you may have one mounted on your helmet or handlebars). I can’t confirm it’s the culprit, but hundreds of GB of data have been processed through that app in my iPad. I Quik-ly deleted that app immediately after the second wipe, after restoring from backup. I’ve been tracking system data since deleting that app, it’s remained stable for over a week now.
