Download plugin from wordpress itself.
Add google drive as remote storage. Wordpress will upload backup to google drive automatically.
Your workflow is not correct because according to updraft it will be migration from domain.tld to sub.dom.tld which is not provided in free version.
Here’s what you need to do:
- Open your domain vhost file
- Add sub.domain.tld in that vhost file right after domain.tld (Example: server dom.tld www.dom.tld sub.dom.tld)
- Restart nginx
- Now try to open your website with sub.dom.tld
- It will open and redirect to dom.tld
- Now in wordpress admin panel, Goto Settings > General and change domain from dom.tld to sub.dom.tld.
- Now open your website with sub.dom.tld and dom.tld. You will see website at sub.dom.tld
First perform these steps then let me know. I will tell you other steps if needed.