If we look at the efficient causes as a string of events along a closed loop, we can see that it is possible to have efficient causation without the need for the uncaused first cause.
I think Aquinas' argument breaks down when he posits that there can be no infinite (circular) regress. That assumption necessitates a stop-gap fist cause which itself cannot be caused, but which violates A's premise that all that exists had to be caused.
If all that exists must have been caused by something prior, then first cause cannot exist, by definition. Aquinas unequivocally ties existence; thus, if you are not caused, you can't exist (on your own); that forces a conclusion that first cause not only doesn't but cannot exist (because nothing caused it to exist)!
FWIW
The causal loop is a common objection. There are two or maybe three ways to answer it.
First question: what got the first one in the loop, started? What was its cause?