No, you logic is flawed here. You're going from nothing can be it's own cause to the first cause is it's own cause to therefore it is no thing.
The first cause is UNcaused. The second part of your argument is a misstatement.
Reason/logic has to conform to observable reality.
More correctly, it has to not be falsified by observable reality - and follow the rules of reason/logic. Theoretical and experimental science does this constantly.
To end this silly semantics game, let's just say that Aquinas suggested that all that exists had to be caused, except the first cause. In that case the first cause cannot exist.
Aquinas' argument is self-refuting, and I am not the only one who says so.
It should be obvious that there is a difference between imagining something and using reason/logic
As long as the reasoning and logic relate to objective reality, and not imaginary abstractions.
And that there are different degrees of "knowledge."
We either "know" or we believe. There is knowledge and then there is belief.
If we reduce what is "known" to it's firmest level, we end up with sense knowledge and even that is not pure. In addition, with that limitation, you and I could not have this discussion
We all have defective, incomplete or imperfect knowledge. No one knows everything. To call what we don't know "knowledge" is presumptuous and deceitful.
Our discussion is based on our limited knowledge and understanding. And our reasoning is equally defective.
[Reason/logic has to conform to observable reality] More correctly, it has to not be falsified by observable reality - and follow the rules of reason/logic. Theoretical and experimental science does this constantly.
Everything we "know" is based on our experience, and—faulty a sit may be—our interpretation of it.