Help setting up VPN (PIA) DD WRT Router

[godfather17]

I picked up a router where I want to install DD WRT. The purpose of this would be to install PIA on it. I have tried looking online for some directions on how to properly get my home setup going but I am running into some issues. Please see the attachment which shows my current home network map. 

I know how to install PIA on the DD WRT router but I run into issues when trying to make sure certain things happen. 

1. How can I access my NAS (attached to an old router which has a USB port. This router has it's wifi capability turned off). 

2. Certain devices (Amazon Firesticks) which are attached to the network via ethernet need to have access to the internet by bypassing the VPN (PIA) so I can have access to Netflix, prime videos, etc without having geolocation issues by being connected to the VPN. 

3. Building on number 2. One of my computers is used as a plex server and I use the Amazon Firesticks as clients. This is the major issue. Is there a way to have the Amazon Firesticks access the required files which are stored on the NAS but also still not have issues with geolocation when using Netflix and prime videos. 

4. The Chromecasts, printer, cell phones, and laptops with all be connected via wifi. Will I still be able to cast youtube videos from the cellphones/laptops to the Chromecasts? 

I assume that the easiest thing to do is to have the DD WRT router handle all the DHCP stuff but I could be wrong. Or would it be easier to still use the google wifi router for something such as handling the DHCP stuff. 

Any help would be greatly appreciated. If anyone can point me to the right place where I can get this going it would be great.