Hi @manghig ,
I will provide some suggestions based on my experience.
If you are not using the max-pixels parameter on your images, you should look into using it or setting it to a lower value. It should reduce the picture quality and therefore save some space on your disk. If, for some reason, this setting does not work, you can also look into using different default cameras on your devices (e.g. Open Camera) that allow you to set max sizes or resolutions for your images.
There is no way to bulk delete submissions from the ODK Central UI at this time. I agree with you that it should be a priority on the ODK roadmap, as I also ran into a use case of deleting bulk deleting submissions some years ago. However, if you have the talent to self-host ODK, you should be able to use the API to delete submissions. @yanokwa provided some code that could be adapted to do this in here. There is a soft delete period of 30 days, and I do not know how to work around that.
I believe the “delete submission attachment via the API” you are referring to is clearing a submission attachment. I have not tried this myself to know whether it deletes data or just the reference to the data.
In general, I think ODK Central wasn’t designed for emergency bulk deletes. Typically, you would want to test and monitor your installation so that you don’t run into surprises with data sizes, and if you do, it expects you to be able to expand your disk size to support the larger database sizes. As you mentioned, if local disk storage isn’t a suitable solution, then a S3-compatible storage should work. If all these options are outside your organization’s technical expertise, I would suggest managed ODK hosting provided by getodk.org or finding a hosting organization willing to provide managed ODK hosting for your needs.
Best of luck!