If not that, then are there are self-hosters out there who have documented how to run Nextcloud in a cluster so any one server going down doesn't disrupt operations?

The closest things I could find were:

1. a database cluster (MariaDB), a shared filesystem (glusterfs, lvm?, cephfs?, zfs?), a couple proxies, a controller, and a ton of manual setup

https://severalnines.com/blog/deploying-highly-available-nextcloud-mysql-galera-cluster-and-glusterfs/

or...

2. Some magical thing to "sync the database" (which sounds like a read-only mirror at best)

https://help.nextcloud.com/t/how-to-efficiently-mirror-backup-nextcloud-primary-server-with-the-secondary-one/149977

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Update: I found https://webhosting.coop/ and while it doesn't look like they host Nextcloud instances now, there's a chance they might be able to be convinced to do so in the future.