Network Design with 20 remote buildings dark fiber and MPLS

I'm working on a design for remote buildings and need some advices. I have 20 remote buildings with dark fibers between them. The physical network design of the fibers is currently based on the distance between sites. So we have rings and multiple path.

So we need to bring network connectivity to these buildings and none of them will have a direct internet access. We will centralize and share the internet access in our datacenter.

On 6 of these remote buildings, a network provider has installed some HPE with MPLS. The MPLS is used with VRF in order to be able to separate use cases.

I'm not familiar with MPLS and it sounds like a complex solution (if we have to manage and deploy it), so i would like to have your thoughts, what can i use between with buildings (no internet access, only dark fiber, ring topology) ?

We want to have connectivity to join our datacenter, be able to manage differents uses cases :

• workstation1 managed by our team
• workstation2 managed by a different customer
• VoIP
• DMZ per building
• Internet
• ...

I looked at SPB / TRILL / Fabric / 802.1aq which seems to be simple but i dont really know how to handle the customers ? VLAN ? VPN ?

SD-WAN looks "cool" but we dont have internet on each building so i dont really know which solutions could apply

Basically we want something with centralized management, simple and cost effective :)

If some of you have any recommandations ? Is MPLS the only way to go here ?

Thanks!