I like to code, garden and tinker
It’s still a fingerprint, the most vague information correlated with other data points can make a useful fingerprint. This is how a lot of the companies can track you even if you aren’t logged in, you using any service creates a pattern that with enough aggregate data can be used to approximate who you are.
Each instance serves the content from that instance, so from my understanding the only thing other instances can see are subscribed communities to be able to federate posts. Upvotes/Downvotes can possibly be tracked per user as they are federated on a per-vote basis currently, though this is just something I read and don’t have sources at the present.
Being an admin of an instance, I can’t even see my own history of visited posts. I can’t verify this, but I doubt this information is being stored in the database currently.
This being said, each instance has full control over their API server and the web-based application being served, so they could add monitoring to either to gather this data. If they did this on the API end it would be undetectable. Running your own instance is the only fool proof method, otherwise you need to trust the instance operator.
As for the data transfer costs, any network data originating from AWS that hits an external network (an end user or another region) typically will incur a charge. To quote their blog post:
So you won’t be charged for incoming federated content, but serving content to the end user will count as traffic exiting AWS. I am not sure of your exact setup (AWS pricing is complex) but typically this is charged. This is probably negligible for a single-user instance, but I would be careful serving images from your instance to popular instances as this could incur unexpected costs.