Learning as I go.
1. It occurred to me that while the composite planks will be anchored to the joists, they won’t be screwed down onto it. Therefore, I needed more lateral stability. Added.

2. Butyl joint tape is heavy and extremely sticky, so laying it on the joists takes some technique, which I *think* I’ve learned: Weigh one end with a brick, roll out the length needed along the joist, then cut. Lastly, remove the backing and slowly work down the joist, pressing the tape as you go. Whew.