Network design question

We are planning to move few of our environments from the office server room to a datacenter, need to expand, but there is no more available physical space, cooling capacity issues, etc.
Number of office networks are currently all interconnected via SonicWall firewall acting as an internal router .
The plan is to use a metro layer 2 link from the office to the DC creating extension of the local 192.x network
The problem is that the Internet connection and few internal networks will stay in the office while other networks, including our development (10.x) network will have to be migrated to the DC.
Here comes the question: the only router that routes traffic is going to stay in the office, means there is no way for the office users to reach the 10.x subnet.
We can install additional router in DC and create static route on office router to forward all traffic coming to the DC networks to the DC router, but I am not sure if this workaround is going to work.
Another option is to get rid of the 10.x network and flatten the at least two of these networks (192 and 10) but considering the amount of work associated with this way it may take a few months of planning and executing this plan, something we do not have resources for.
My goal it to try and keep network design as simple as it possible.

What can work in our case?

Thanks!