Thanks a lot for the reply! It makes a lot of sense.
You are quite right in terms of the domain setup - SEO is a major part of setting up the blog, and has max effect if it’s on a /blog address.
If a bit of complexity and cost is necessary that is ok. However, what kind of load balancer would you suggest? Google Cloud Platform’s own load balancer doesn’t seem to do the job, as the Firebase hosting is not directly hosted within GCP…