Regarding options:
I’d say this is best short-term measure if things are tight. But not as a long term solution - more instances are needed (as I’ve tried to emphasize in my Panoramax talk in SotM-HR just few days ago).
You could ask for donations for that purpose specifically (like here in this thread!), and I’m sure some people would be willing to pitch it (myself included; even if I have very few pictures outside Croatia).
Perhaps also contact contributors (at least top ones) of imagery outside France directly, and make them aware of the problem? It is likely that many are not aware of the issue, and will consider other alternatives thus reducing further (unsustainable) load on OSM-FR servers.
- do not accept pictures outside of France anymore
Understandable, but it would be quite a blow to the project to lose the main fallback Global instance IMHO. But it the extra storage does not help enough, then it is only acceptable remaining choice.
- delete pictures from some accounts who did not follow the “testing only” rule (or move them to new local instances)
I would plead not to do this one.
Firstly, that “testing only” was not always clearly communicated (e.g. there were plenty Fediverse posts asking people to contribute pictures and improve growth of users, pictures, kilometers; without any mentions that stuff outside of France will be deleted in few months and efforts wasted unless non-OSM-FR instance was chosen)
Secondly, destroying existing data that users have uploaded in good faith would be a public-relations-suicide move for the project as a whole IMHO. (remember what the much smaller CC-BY-SA vs. ODbL license-schism and resulting data removal almost did to the OSM project?)
I’d instead recommend existing OSM-FR uploads be grandfathered in, even if new contributions outside France are forbidden. Perhaps directly contact those contributors which use most disk space outside of France and help them to find a solution?