Ugh... I don't want to denigrate your advice (which I'm sure is great) but at this point in the comments we're getting so far away from the, "keep it simple" approach in the article that it's getting ridiculous.
If you're at the point where you're concerned about cold starts and using a service like fly.io why not just skip the (unbelievable, no-one-knows-this-stuff) complexity and use a $5/month VPS?