Thanks for the feedback - I'll definitely include which imagery was used in future similar changesets' tags!

I was using the King County Orthoimagery as my "authoritative" alignment for a bit (and still use it very often, especially because the 2021 imagery was taken when many trees hadn't grown back their leaves) but received a message from another prolific Seattle-area mapper saying that it's better to use Bing as a reference and that is the local convention, so I've been following that since then.

In JOSM, I set the offset for the 2021 King King County Orthoimagery to "-1.7915,0.8959"

I also wanted to try recording some GPX tracks over time and figuring out a "perfect" alignment (but the hills make that impossible) however I only have my phone's GPS, which I can't really rely on for perfect accuracy.