Sharing resources between ISP + WDR4300 routers

So, instead of just using my WDR4300 and WAN, effectively just using the ISP Huawei HG8247Q as a fiber modem for my 100/100 connection, and thus overloading the WDR4300, I'm trying to get the best of what I have available.

So my objective would be sharing the workload between both routers, namely the following:

ISP router (Firewall, DHCP and UPNP disabled) would handle:
5Ghz AC Wi-Fi
SMB Share (Handles NTFS @ 15Mb/s)
DDNS Client

2.4Ghz Wi-Fi (better range)
DNS (Rebind protection disabled, otherwise IPTV Box won't work)


Now, I already have this working, WAN port is working as LAN on same subnet, but I have some questions about some configurations, namely:

How should the Firewall be configured? I'm guessing that since the LEDE firewall is working only on LAN rules, and ISP router has the firewall disabled, I'm completely open to the WAN, right?

Just deleting the WAN and WAN6 interfaces is enough? Or should I not even delete them?

Are there any particular rules I should be aware for things like DLNA/Multicast over OpenVPN to work?

Is there any way to use SQM in this config?

Are there any performance tweaks I should apply?

Thanks in advance.