It is almost impossible to favor a certain application for download bandwidth. You can do little if the link is overloaded and the ISP discard a packet. You can not get that packet back no matter what you configure.
The only thing you can truly control is outbound traffic. Obviously you could block all request to any site other than steam and netflix and they would then be left with all the bandwidth.
Hard to say if you do anything at all with this router. The only ones you even have a chance on are ones that let you set certain groups of traffic to fixed levels. The high.medium,low crap is mostly for show and all the DSCP markings that some routers have are stripped off as soon as the data gets to the ISP.
You best option if...