The number of users is not really what drives costs honestly. Or at least, it's not like a linear relationship. I think actually having many popular communities might be a bigger issue.
Yes, storage costs matters. I think it's honestly crazy that Lemmy caches images as much as it does. It would be great to be able to just disable it completely, but alas you can't do that without disabling uploads for your own users either (at least I don't know how).
Disabling uploads might be an option. You probably just need to announce it to the people beforehand, but I see most of the people using image links from other sites, so that might not be an issue.
Yes, but honestly unless you're very big, federation queries are the bulk of the processing and stuff from your own instance doesn't matter that much. I mean think about it, do you think the 100 active users on your own instance is what costs or the 10000 users posting all over the fediverse is what matters? Obviously the latter. So again, local user count is not that impactful.