A small nuclear warhead might kill poison ivy. It seems the best way is mechanical removal, but you're going a long way toward weakening it with your layered approach after your initial removal.
I suspect that you'll still be digging out a few runners in the future, so don't let them get re-established. Give the layers of stuff a try, and when you get ready to plant, don't for a while. Let any remaining hidden poison ivy show itself, and go in and physically dig it out if you see any. Then plant the bed how you want. If you can get those underground spreader roots out, you'll be fine.
Good luck. I've slowly pushed it out of the back of my yard, but it isn't gone, it's just waiting till I stop watching for it.
_________________ Northwesterner
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