I’ve read the cloud engineering headache write up from a few different Lemmy servers now so I no longer can remember which ones are on CDNs and which ones aren’t. Are you backed by anything in particular? Once you are on object storage you are likely to find a surprising egress bill if you aren’t caching the images and other content at a higher CDN tier.
You should also more or less immediately get lifecycle policies setup in your object store to tier older posts into cheaper storage. It’s 10-20 minutes of learning effort that stands to save you thousands of dollars. The trade offs with storage classes are usually cheaper storage as reads get more expensive. Since Lemmy promotes fresh content there is likely a balance you can strike in your lifecycle policies to drive storage costs down for older content that is unlikely to be loaded a ton of times.