The universe is vast and complex. So it follows that its creator must be even more complex. To paraphrase your statement, it seems implausible that we should believe a creator exponentially more complex than a watch could could just be.
Einstein was of a poetic bent. But, further, he did real science. So whether he was theistic or not is really irrelevant. His science is what that matters.
Form, function, purpose, design are hallmarks of a creator.
And you assume that all these things can be shown in nature. They cannot. Further, those that can simply does not point to any creator other than natural forces.
It seems implausible that we should believe that something exponentially more complex than a watch (to use the old but pertinent analogy) could come into existence due to mindless natural causes.
You misuse the word "implausible." It is very plausible. Further, here you mix apples and oranges to reach the result you want. The complexity of a man-made, mechanic object cannot be compared to the complexity of a biological organism with any reasonable hope of gaining any understanding of either. It is like dancing about architecture. The two just don't mix. (And, at its very heart, biological matter is rather complicated, but simple.)
It seems more reasonable to believe that there is a mind, an intelligence, a creator.
It does not seem reasonable to me to believe in something--to base your life and your entire outlook on life--on the existence of something for which there is no proof nor evidence.
Where did God come from? A God with the power to create the known universe is beyond our comprehension. Kalam's argument says that everything with a beginning had a beginner. Scripture reveals God to be infinite, without a beginning. It is outside our experience.
But that argument, again, is based on an a priori belief in that God and that scripture. There is no logical or rational reason for either of those beliefs over the alternatives.