Setting up Fritzbox 7530 behind ISP router as VPN router

After trying for some time to figure out on my own, I am now at a point where I seek help because there seem to be some things I don't understand.

I am behind an ISP router (192.168.178.1) that is needed for VoIP.

Behind that I have a FB 7530 with OpenWRT 23.05.3. (192.168.1.1)

First hurdle is how to configure my device so I can use LAN port 4 as my WAN port, as I won't use the DSL function of the router and the FB 7530 doesn't have a native WAN port. I am not sure if I already need to configure this as a VLAN setup to systemically separate ports. But as my network anyways needs more VLANS for other spaces it's probably only a question I'd like to understand.

Second: My OpenWrt router is meant to send all traffic through Mullvad VPN for privacy reasons. So people can choose between using the ISP router without VPN (better speed less privacy) or with VPN without needing to run software on the devices themselves.

So far I was stuck at the point where my OpenWrt config was able to work as a normal internet access point. But the FB itself was not able to ping any site or update itself through opkg.

I've just noticed the drawing has some minor mistakes. VLAN1 is meant for iot devices. VLAN2 is connected to a hardware switch with more LAN ports as all 3 free LAN ports will already be used by the VLANS.