What he said. Cake is designed to handle most situations without config, thouge some of the possible tweaks can help this situation, see the more advanced sections of the docs on the OpenWrt site.
And, to the rest of your questions, there is a wealth (well maybe too much wealth!) of info on the prioritizing of packets in this thread: https://forum.openwrt.org/t/ultimate-sqm-settings-layer-cake-dscp-marks/25832
There's so much in that one, though, that it would be a Good Thing(tm) if a few of the knowledgeable folks goes thru it and mines out stuff into a nice instruction page on the various topics and methods...