Asuswrt-Merlin Dual WAN setup question

[JMan7]

Hi,

I need to be able to connect via VPN to a remote AVM Fritz!Box 7590.
Fritz!Box's support only pure IPSec VPN.

My primary router is a Asus RT-AC88U with Asuswrt-Merlin and I would like to keep this as my primary router.

I have a AVM Fritz!Box 7490 which can create the IPSec VPN tunnel to my remote AVM Fritz!Box 7590.

I'm having the following setup in mind by using the Asuswrt-Merlin Dual WAN setup feature but would like to confirm first if this will be possible .

Thx a lot.

Problem:

All network devices/clients connected to the RT-AC88U / 192.168.3.x need to be able to connect to the AVM Fritz!Box 7590 network devices 192.168.1.x.

Idea:

Setup the AVM Fritz!Box 7490 as a second WAN provider in the RT-AC88U. Then route all traffic to 192.168.1.0 via the second WAN only.
Any other traffic should be routed via primary WAN.

Question:

How to setup the Dual WAN on the RT-AC88U as the types are only Load Balanced or Fail-Over?
I’m worry that when I setup Load Balanced that also other traffic than to 192.168.1.0 will be routed via the secondary WAN.
I mean I will setup the AVM Fritz!Box 7490 anyway to only route traffic to 192.168.1.0 via the VPN tunnel,
but it would be preferred if the Asus router already can ensure the correct routing .