I think a good approach to defining consciousness is to ask what problem it solved. Why was it selected for.
The obvious functional purpose of consciousness is self-awareness. Awareness of our body in our environment, via our senses, and awareness of our bodies internal state, and all the way to awareness of our primal mental state (feelings, impulses), to awareness of our highest level mental states: what we are paying attention to, what we just deciding that is already impacting our bodies behavior, etc.
The more aware we are of ourselves in the center of our world, the better our ability to control our responses, and learn from the results of our responses.
The point where we are functionally aware of our own thoughts, including that we are aware of our own thoughts, and have enough ability to abstract to experience that as a closed infinite loop (as an abstraction) in real time -> that's consciousness.
The only thing mysterious about it, is we have that strong loop of awareness, but without awareness of a lot of the machinery under it that makes it possible. So to us, it appears to float in out minds free of actual machinery.
But just like we experience memories, without complete awareness of how they are created or stored, experience pain, without experiencing the nerve pathways that took a sensory input and passed it on to our pain center, we are aware of our own thinking about out thinking, but not aware of the machinery that makes us think.
Anyway, to me the functional, behavioral, benefit of the design approach seems sensible, since evolution is what produced our consiousness. And the answer I take from that seem reasonable.
Consciousness is just the depth of our real time awareness of our real time awareness.
And I would expect machines to get this, since this highest recurrent level of internal awareness provides the most information for self-regulation, control and adaptation.
But it seems like something that is easy to withhold in models too. No tight realtime feedback of highest level internal states. Intelligence, with enough self-awareness to be able to function sensibly, but with low or no internal stream of consciousness and the experiences associated with that.