Do you need Mastodon, or can it be something similar?
Mastodon does a lot of caching.
For my single-user instance I run Akkoma, which is very leight-weight and has all (even more) features Mastodon has.
Well that doesn't differ much from what I do. I just copy the files to a second directory and test with that. Easy rollback. Downside is, that all data is lost between upgrade and rollback, which will be the same in the scenario you suggest.
Technically I could. But as with mastodon.world I have made a promise to ensure continuity by creating a team of sysadmins and moderators with access, and making sure it can be transferred to them might I get hit by a bus, or get bored of this.
But I hope there will be a feature to export your posts soon so you can create your own backups too.
By the way this is Media disk usage for my Mastodon instance, but it's not really small ;-)