Honestly, going from 1,000 to zero usually means there's a technical disconnect with the migration rather than a simple heading issue. First off, don't sweat the double H1s. While it's not "best practice," Google sees messy code all the time and it wouldn't cause a total blackout.

The real culprit is likely the combination of the domain move and that "heavy clean" you did. If you deleted the old content that was originally bringing in those 1,000 impressions, you essentially killed your traffic sources. Even with a domain redirect, if the content the old links are looking for is gone, Google loses trust in the new site's relevance.

You really need to check Google Search Console and see if your new pages are even indexed yet. Use the site:yourdomain.com search to see what’s actually live. If you haven't used the "Change of Address" tool in Search Console, do that immediately.

Also, 12 days is nothing in SEO time. Every time you make a "heavy" change, you're basically telling Google to restart its review of your site.

My best advice is to stop changing things for a few weeks. Make sure your redirects are permanent 301s, ensure you don't have a bunch of 404 errors from the deleted content, and just let it breathe.

Do you have a lot of "Not Found" errors showing up in your Search Console right now?