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I don't this question is generally addressable without discussing your meta-ethical views.
If welfare is good because (and only because) we as individuals care about our welfare (call this Premise C) then things being good requires actual people (whether past/present/future) to exist in the first place - otherwise there is no source of value, and no basis on which to judge what is good and what is not.
Note that this isn't necessarily constructivism, insofar as you will have something like "X is …
One challenge to Premise C is the temporary depressive. Suppose Adam doesn't care about his welfare, and is considering suicide. He also has access to a 100% reliable antidepressant pill. If he takes the pill, he will have a very happy future (which he would, at that future time, highly value). But right now, he doesn't value that at all. So he commits suicide instead. Did Adam make a mistake? I think yes: he should've taken the pill and had a happy future instead. C implies no.
For furthe…
The Adam case works as an intuition pump, but it falls short of being an argument. For someone inclined to accept C, it's pretty easy to accept that Adam didn't make a mistake.
On the moral realism thing — it may be true that your normative claims can be separated from your moral realism, but your writing style is so pervaded by realist turns of phrase that it's easy to see why people keep picking up on that and assuming that your realism is central to your argument. For example:
>"it’s insane to deny this premise" >"important moral insights" >"any theory formulated in purely negative terms... cannot possibly be correct" >"moral theorists have misgeneralized their intuitions"
Whether or not this matters depends, I suppose, on how persuasive you want your arguments to seem to readers who do not share your moral realism.