OPNsense instead of Ubiquiti UDR as main router

[AGTre]

Hello! New to the forums. 

I feel like I'm in need of some advice with this particular case.

I have two houses, but fiber is only available at one of them (Ubiquiti airFiber to the rescue).

At the moment the UDR is the main router (and controller).

The problem with this is that the UDR is not nearly powerful enough to handle the traffic if it approaches 3-500 download.
Even the interface becomes totally unresponsive, so when I sit in the remote apartment, which I always do, I have no way of finding out what the problem is, or even doing anything about it.
I even tried turning off Threat Detection on the UDR, but same thing happens.
This was not an issue on the USG I had previously.

So, I then remembered buying a 3rd party device with 4 ethernet-ports, a 4-core J4125 and 8GB RAM a while back.
My thoughts back then was using it to try out pfSense. That was before talking to a friend of mine. He suggested I should rather try OPNsense.

Here is what I would like to achieve:

- Swapping out the UDR as main router with the OPNsense device
- If possible, keep the UDR as the device providing WIFI in that same house, otherwise use some mesh devices
- Possibly transition to a cloud-based unifi controller?
- Some degree of redundancy for the remote location if the airFiber link goes down (hasn't happened yet though)

Current networking equipment:

- Ubiquiti UDR
- 2x Ubiquiti AirFiber 60-LR
- 1x USW-16 150W PoE
- 1x Ubiquiti UAP AC LR
- 1x Ubiquiti UAP AC Lite
- (1x 4-port OPNsense device)
- (1x set of aging mesh-devices (do not seem to work with VLANs))

Local networks:
 
  - Networking equipment
 
  - Local House - Home network
  - Local House - Guest network
  - (Local House - Possible apartment 3)

  - Home server(s)
       - Located at the remote site with the 2 apartments
  - Apartment 1 - Home network
  - Apartment 1 - Guest network
  - Apartment 1 - IoT network
 
  - Apartment 2 - Home network
  - Apartment 2 - Guest network
  - Apartment 2 - IoT network

I'm sorry if the attached chart is technically wrong or not detailed enough.
If anyone has any advice I'd be happy to provide more details though.

Any suggestions as to what could be a good way to set this up?

[smthing]

I think you should specify your question a bit. But presuming a general question "can you use OPNsense as a gateway instead of Unifi?" Sure, you wouldn't be the first to do it either. Basically all "networking" services can be done by OPNsense. It's up to you, where you want to place each service. You can leave the WiFi tasks to Ubiquiti for convenience.

I would start with what VLAN should be where and how my networking hardware would support it.