I believe the Pictrs is a hard dependency and Lemmy just won’t work without it, and there is no way to disable the caching
I’ll have to double check this but I’m almost certain pictrs isn’t a hard dependency. Saw either the author or one of the contributors mention a few days ago that pictrs could be discarded by editing the config.hjson to remove the pictrs block. Was playing around with deploying a test instance a few days ago and found it to be true, at least prior to finalizing the server setup. I didn’t spin up the pictrs container at all, so I know that it will at least start and let me configure the server.
The one thing I’m not sure of however is if any caching data is written to the container layer in lieu of being sent to pictrs, as I didn’t get that far (yet). I haven’t seen any mention that the backend even does local storage, so I’m assuming that no caching is taking place when pictrs is dot being used.