The premise that "Every event has a cause distinct from it" is nothing but an assumption.
An event can, and often is, caused by itself, or perhaps by another instance of itself.
For example, rain is caused by rain. Because of rain there is an accumulation of water which then evaporates into clouds. This causes rain. This is cyclical causality. St. Thomas' argument is rigid.
More importantly, even if we somehow could determine there was a single cause that started everything, that in no way means that this single cause is "god", that is to say it is some kind of an absolute being which has no cause.
No event has a single cause. For every event there are many causes, and each event is a cause of many other events. It takes a big leap of faith to state there is a single cause of all events, and an even bigger leap of faith to state this cause is god.
Extrapolation is always a questionable method of coming to conclusions. Why not just admit we have absolutely no idea of how everything started. What is the use of this speculation? Let's just live the best we can: Let's be kind and gentle, let's avoid harming others and ourselves. Speculating about something that is completely beyond our comprehension is pointless, a total waste of time and energy.
Suppose a programmer writes a computer game named "Universe and Life." What would be the point for the characters of the game to speculate about the reality outside the computer? Sure, the game has a major cause: the programmer. But is "it" a single cause? Of course not. The programmer has been influenced by other games and other programmers, as well as many other factors. All of these are completely beyond the grasp of the characters in the game. So, why even try to figure it out? No matter what "solution" they may come up with, they will be wrong.
St. Thomas was equally wrong.