Switch on Fritzbox 7362 SL not working

Add eth0.2 to the LAN bridge, for example in LuCI:
Network -> Interfaces -> LAN -> Edit -> Physical Settings -> Interface -> Custom Interface -> eth0.2

Cake's ACK filter could also be worth trying, once SQM works.

Most current devices can handle SQM for a 100/5 Mbit/s link. The VRX2xx is rather slow, but interesting for users who need the xDSL modem.

Some benchmarks:

More advice:

Do you plan to use Wireless LAN? Which device is going to be the access point?

Could you put your cable modem in bridge mode, and use the OpenWrt device as a router? It is not required for SQM, but will allow you to use more of OpenWrt's other benefits.