Since you expressed openness to this dialogue, I'll share. When I really, really stretch my moral imagination to the point it decouples from practical logic, I find myself feeling spiritually reverent for the things that the mental abstraction "complexity" points at. I arrive at this by synthesizing a particular metaphor from Alan Watts with Mihalyi Czikszentmihalyi's ideas on complexity.
In the same way an apple tree apples, the universe peoples. It also dogs, and weathers and softwares and cows. Of it emerge these beautiful structures, including plants. If I was capable of surviving the endeavor, cherishing and protecting every single one of those things that the universe expresses would be my ultimate ethical goal. I don't know if there's farther expansive rings of morality. I imagine I might be able to peek at some with the help of deep, deep meditative practice or other tools.
The thing that re-embodies me is the Buddhist idea of bodhisattvas. They are beings who have reached enlightenment, but then rejected paradise in honor of the work still left to do on this plane of existence.
That's basically the outline of my metaphysical beliefs.
Finally, specifically regarding pain and plants, I agree that we bend our intuitions to the experiences that make sense to us. My argument is that the word "pain" and what we understand pain to be, is predicated on a specific biological structure, nociceptors. To speak of pain without nociception is like saying the water in my hand isn't wet. They're enmeshed, dependent properties. So, if plants do experience something that increments the universe's suffering counter, which I leave as a possibility, we would have to imagine some other form of communication. Language and thought extrudes from the human body. As a consequence of that fact, there are just some things it can't capture, and attempting to do so places us closer to the choice: bodhisattva or buddha.