biggest concern is getting all participating instances to agree
I see what you mean, that is true if the responsibility ultimately ends up falling on instance owners.
Which is why I’m hoping that the developments instead occur on the Lemmy project itself and other fediverse project code bases. Lemmy devs and contributors will hopefully work on privacy and security as the Lemmy project matures. If instance admins are keeping their instances mostly up to date, there is virtually no (dis)agreement to be had: the mitigation patches will be loaded on the next update.
Of course, anyone can fork lemmy or manually remove these changes from their instance, or some admins may simply refuse to update, but that would reflect badly and privacy minded users may choose move to another instance that has updated to the latest/most secure version of Lemmy and other instance owners can also choose to defederate from instances that leave themselves vulnerable to issues that have been patched out.