Migrate off Dream Machines to USW-Pro-Aggregation, Cloud Key+ and NVR?

-The Problem-

I'm currently in a new role. It's a weird set up and there are some layers of complexity. We would like to reduce the layers of complexity.

Right now, in this environment, we have a mix of Fortigate, Dell Core switches and Ubiquity Dream Machine Pro's along with Ubiquity Layer 2 Agg switches (USW Aggregation) and USW-Pro-Max-48 switches.

From what I can tell, they are using an USW Aggregation switch to pass internet directly to the Dream Machine Pro's? They are using the Dream Machines essentially as a controller at this point. They have told me that the Dream Machines are in "passthrough" mode. I dont see a way to turn off any of the firewall or routing functions of the Dream Machines.

The other USW Aggregation and USW-Pro-Max-48 switches are connected directly to the Dell Core switches.

On top of all this, there are VLAN's defined in the Fortigates that are being passed through to the Dream Machines. There are VLAN's and scopes defined in the Dream Machine as well.

With the Dream Machines set up in this way, it's an added layer of complexity and I feel they weren't really designed to be used this way, especially in a heavily VLAN'd environment.

This is how it's setup currently:

Internet uplink >USW Aggregation > Dream Machine Pro passing 2 VLANS.

All other Floor and Access switches > Dell Core Switches.

Fortigates (passing 4 VLANS)> Dell Core Switches > Dream Machine using mDNS?

-The Proposed Fix-

What I think should happen, to migrate off the Dream Machines, is install a Layer 3 Ubiquity Switch (USW-Pro-Aggregation) and migrate to a Cloud Key + for control of switches and access points. Then, I think, I can migrate door control and cameras to the NVR. Then I can move the Ubiquity switches access switches to the USW-Pro-Aggregation. Then form a trunk to the Dell Core switches from the USW-Pro-Aggregation. I think this should work and give us visibility and take away the weird VLANing.

Not sure how to go about this since everything is all in production at the moment.

How would you fix this?