Probably mobile+desktop is reasonable.
I often resize when I have no device selected, but when a device is selected, the current RDM removes all the device DPR/user agent overrides when resizing the viewport, which is really annoying.
I like all my controls to be visible. However, it might be nice to simply be able to customize what you want to see like in Chrome.
Not related to any of your questions, but I would love to have a way to change the user agent string independently from the device. The old XUL version of RDM used to have that ability.