You have correctly identified that you only have a layer 3 path between your new switch location and the current layer 2 location that you want to extend.
This leaves you with only two options:
1-Add the link as you note and create a layer 2 path that you can transport the vlan between the new switch and the MX
2-design and create a layer 2 service that can use the layer 3 transport to connect the vlan between the new switch and the MX