Thanks for the clarification on your stance and for sharing the link. Although I know a lot of people think Dawkins worships on the alter of rationality, I think some of his stances on this issue need to be taken with a grain of salt because he openly admits that he occasionally makes emotional arguments when it comes to (non-human) animal rights.
I think the assumptions that need to be called out in Singer's argument (and possibly yours, given the little bit in your post above) are 1) we know how to define sentience and 2) we understand the link between sentience and suffering. These points may be a departure from Singer in part because he's making a philosophical argument and not necessarily a scientific one. The science part being relevant to this thread because I think part of the discussion is how strict definitions are necessary to avoid the moral relativism aspect of the conversation.
There doesn't appear to be any scientific consensus on how to define consciousness or sentience. Some, like Sam Harris, define it simply in terms of information processing. In that case, what suffers can get murky fast. Are mollusks more conscious than, say, a venus fly trap? I don't know, but as a layman, I haven't come across a good definition to parse that question effectively.
Even if we have an understanding of sentience, there doesn't seem to be a definition on how this translates to suffering. You seem to imply it is a given that sentience leads directly to the ability to suffer, but I haven't found a good consensus on this either. Some people like Gerald Edelman take the stance that suffering is dependent on complexity of the nervous system, so there's nothing morally wrong with killing lobsters and less complex animals. There's others like Vilayanur Ramachandran who go further and don't think more complex animals like cows suffer because they don't introspectively experience pain the way humans do.
The above is a just a long-winded way of saying that I think we need strict scientific definitions to make some sense of the topic or we'll just be endlessly entangled in relativistic arguments. I'm not convinced we have made sense enough of the problem to define it this thoroughly.