"What is the First Cause?" is not the same question as "Why is there anything?".

This is true in the absence of further assumptions. But once you or I assume Maitzen's argument is true (and I think we both do) the second becomes a mere instantiation of the first.

An infinite string would answer the former, not the latter.

This is false if Maitzen's argument is true. Conditional on Maitzen's argument, an infinite string answers the former and hence the latter. I could justify this by repeating what I've written in my two comments upthread, but it might be more productive if I give a different argument.

How do we usually answer "Why is there X?"? I think we usually pick out X's most salient cause. "Why is there an ambulance outside my neighbour's house?" "Because the neighbour had a heart attack." We're basically saying, "here's the most interesting antecedent node in the causal graph, and had we deleted or substantially altered it, there wouldn't have been X". If we'd deleted the "neighbour's heart attack" node, there wouldn't be an "ambulance outside neighbour's house" node.

This gives me a way to interpret "What's the First Cause?", or "Why is there anything?", or "Why is there everything?", or "Why isn't the universe in the counterfactual no-turtle state?" (to paraphrase you). These questions are asking for a node in the causal graph that's antecedent to everything. But how can I do that if the causal graph is an infinitely long string? There's no such node!