LXC isolation protects Proxmox from container escapes, not services from each other over the network. Full disk encryption protects against physical theft, not network attacks while running.
And if Nextcloud has passkeys, HTTPS, and proper auth, what is Tailscale adding exactly? What is the point of this setup over the alternative? What threat does this stop that "hardened Nextcloud, exposed directly" does not? It is complexity theater. Looks like defense in depth, but the "layers" are network hops, not security boundaries.