I’m sure this will turn out to be a stupid question, but what about just getting a DigitalOcean server with lots of storage? They have very large transfer allotments as well (in the multiple TB depending on the droplet size).
Off the top of my head, my guess is that if everything is served from the same server it might slow down the site. Does this make sense?
Also, the other obvious downside is that you would be paying for storage you aren’t using until your user base grows. But the S3 transfer rates are so high that you would probably still come out ahead over time.
Again, looking for people to poke reasonable holes in this suggestion as we are all trying to find the right balance.