An option to set a cache limit would be nice. Caching is okay, as long as you can clear the cache. Something like Mastodon caching. You can prune all remote media via tootctl
Turning completly off will increase the traffic of all other instances -> longer loading times 🤔
A new Subdomain is like a whole new instance. I would not move over old posts or something else.
Start completly fresh and have fun instead of headaches because of broken Posts 🌞
You can try your setup with another subdomain. This way you will see If there are any problems
And never lose your database. It is the heart of your instance, so do proper backups ☝🏻