VPN Router in front of RBR50 +2 Satellites will it work, need to know tricks

Future config:   Cable Modem <->  VPN Router (Asus RT-AX88U w/Merlin Firmware) <-> Existing Router (Orbi RBR50)

I'm thinking about adding a Asus Merlin RT-AX88U Pro as a front end router to provide VPN coverage for all of my home network that is connected via an Orbi RBR50 AC3000 Mesh router with 2 Satellites.  150+ devices in total.   

My concerns are several.  Are there any specific config settings that need to be added / changed in the Orbi so all traffic flows correctly through the Asus VPN router?  Is there any need for VPN Passthru which is not really supported, though port forwarding might work?  

The Merlin Client has alot of VPN capabilities so configuring IPVanish should be simple.  I will also need to unblock several services that need to see my native IP service, and this is easy to do as well. 

I'm pretty sure that I will need to change the default IP Net to something other than 192.168.1.x on the Asus so the Orbi can just be plugged in and seen without having to reassign any addresses which would be a really big problem with all the automation and managed devices.  

Last I'm open to any tips or tricks learned in this type of setup.  I'm not a networking guru by any means, just enough knowledge to get myself in trouble, so looking to prevent it if possible.