People distributing unmentionable content or using services for otherwise nefarious purposes are used to dealing with intermittent issues like that. It'll get rid of some but not all of them.
> c) expire & purge the (encrypted) content quickly
If your site turns out to be being used by a group that is raided, this will not stop your services being confiscated and very thoroughly investigated. You might be legally in the clear, but it may cost you time and hassle (and potentially money if you need to take on the services of a lawyer to help prove you did not intentionally aid/abet).
Also an end-to-end encrypted solution like that might be rather attractive to users both illegal and otherwise, so be ready for a huge bandwidth bill if/when a sizable group latches onto it!
If it is mostly intended for personal use, you may as well use some form of access control. We are long past the day when people would abuse such a service to share distasteful material and in an era when people would abuse such a service to share (rightfully) illegal material. While I was uncomfortable with the former, I certainly do not want to facilitate the latter.
Put the service behind a personal Tailscale tail net. Can still access from all of your devices and nobody else can push or pull.
Could crib notes from olden days and have a public/ subdirectory where files you drop in there are available read only to the public internet via separate endpoint.
Can also have a blind drop box that does the other direction (but does not allow the public to read).
That way you are read / write from all your devices, can publish to anyone, and can let anyone share things to you, you just don't facilitate anyone sharing to anyone.
a) unreliable (e.g. having opening hours a.k.a. intentional downtimes)
b) applying client side encryption a la https://privatebin.info
c) expire & purge the (encrypted) content quickly
would have mitigated your problem.