I would recommend Backblaze B2 storage instead of removing post history.
The problem with post removal is that it will affect all other instances’ as you don’t (by default) cache original images from other fediverse instances (iirc)
So for example, my instance Reddthat will reference an image hosted on your instance, which will then 404. (But the thumbnail will be cached (I think))
Backblaze is dirt cheap and the pictrs object migration is now multi threaded making it very easy to migrate now. I was hosting 15GB and it amounted to bills of cents! $5/TB or $0.005 per GB. If you add CloudFlare Infront you then get free egress! Or any CDN that is part of the “Bandwidth alliance”. I now have ~120GB of pictrs storage. After 2-3 months.
Also, there was a recent chat on the lemmy admin matrix channel about setting some variable in pictrs so it does not cache any images. But I don’t remember what it was so maybe check the pictrs documentation.