Although this is much more expensive than Lightsail, the man hours saved will make it worthwhile.
And each? Surely chrooting users would let you consolidate all of those servers into one (or one cluster for HA I suppose).
Think about the operational costs: someone needs to manage keys, logging, security updates, when S3FS coughs a lung and hangs you need to catch that problem and remount it to restore service, etc. This service reuses the existing authentication systems so you don't need to spend time configuring and managing integration with your customers’ LDAP/AD infrastructure, etc. If you deal with anything which hits PCI, HIPAA, etc. you need to be able to certify that your custom design meets those requirements as well.
That's not to say you can't do it yourself but for many places there's a fairly significant amount of work where the cost of doing it yourself is greater than 5+ years of managed service costs.
We're more interested in what happens when things break (and who's responsibility it is) than minor cost savings in calm waters.