To be honest, I think whichever approach you take is unlikely to have a significant effect on how much energy your website uses overall.

For example, servers in datacentres are very powerful and are able to run more than one thing at once. So if you were hosting your own Lemmy/Mastodon instance, there'd be no reason why you couldn't also host a standalone website on that same server. The difference in energy usage would be negligible.

In contrast, you could argue that Lemmy is less efficient than a straightforward static website because the content of your blog posts will inevitably end up being federated to many other instances. That means multiple copies of your blog will be transferred between multiple servers and stored on multiple hard drives, etc. Whereas a static website lives in one place and doesn't end up using so many resources.

At the end of the day, whichever you choose will likely have very little impact. So I wouldn't worry too much about your blog's green credentials.

I'm saying this as somebody who is pro protecting the environment, but also pro prioritising our efforts in the places they'll have greatest impact. You'll probably have a bigger impact by walking to the store instead of driving.