Having a RTT on cross bars instead of a full rack is not only possible, but considerably easier. Reaching underneath a rack to tighten RTT hardware is a pain. Some racks have custom options for different mounting solutions, but they're all expensive and more complicated than the process should be.
There are plenty of options for mounting awnings to standard cross-bars, too. Brackets
like these are commonplace from different manufacturers. You can bolt them down to your cross bars (much easier to do w/ aftermarket cross-bars than OEM ones), or u-bolt them to virtually anything. The only exception would be awnings like the AluCab Shadow 270 degree, which requires a very strong foundation in the corner/pivot, and would thus be better mounted to a large, rigid rack.
A platform rack makes it easier to mount/carry
small items that are otherwise difficult to secure to cross bars. Things like propane tanks, a single bundle of firewood, jerry can, etc. But the rack 1) costs a lot 2) is heavy 3) limits access to the roof for cleaning or mounting things on the bottom of the rack/cross bars, 4) frequently introduces more wind noise than cross bars.
And worst of all, as soon as you put a RTT on the rack, the tent will take up 99% of the rack's space, so it won't be available for anything else, and having it becomes largely pointless.