GETVPN (Group Encrypted Transport VPN) is a Cisco proprietary technology that provides dynamic encryption for spoke-to-spoke communication over a private MPLS network. It does not require the maintenance of permanent tunnels, and it uses dynamically changing keys.
DMVPN (Dynamic Multipoint VPN) is another Cisco proprietary technology that can be used to create a VPN over a private MPLS network. However, DMVPN does require the maintenance of permanent tunnels, and it does not support dynamically changing keys.
GRE VPN (Generic Routing Encapsulation VPN) is a generic VPN technology that can be used over any type of network, including a private MPLS network. However, GRE VPN does not support dynamically changing keys.
Standard IPsec VPN is a generic VPN technology that can be used over any type of network, including a private MPLS network. It supports dynamically changing keys, but it requires the maintenance of permanent tunnels.
Therefore, the best choice for this environment is GETVPN. It meets all of the requirements, including dynamically changing keys, spoke-to-spoke communication, and no need for permanent tunnels.

In the Software-as-a-Service (SaaS) model, users rely on the service provider to manage the infrastructure and applications. While this reduces the burden of maintenance and upgrades, it also limits the control customers have over the underlying infrastructure and applications. This lack of control can be a challenge for organizations that need to customize or manage specific aspects of the application or infrastructure to meet their unique requirements.

You have an IS-IS router that is performing both L1 and L2 routing and has both L1 and L2 neighbors. How would you optimize the router's operation to conserve bandwidth and router resources?
Configure each interface as either L1 or L2 circuit type, depending on the type of adjacency needed out that interface. The command to do this is, at the interface configuration mode, isis circuit-type [level-1 | level-1-2 | level-2-only]. This prevents unnecessary hellos from being sent out interfaces, which uses bandwidth and router resources.
https://www.ciscopress.com/articles/article.asp?p=101756