If one acts out of "regard for one's own concern for a cause", one acts out of self interest.

I disagree.

The above notion of self-interest is a dictionary.com definition of self-interest and interest, but not the only logically conceivable definition mind you, I don't need that. So it is still possible for that notion of self-interest to contradict another potentially acceptable definition of self-interest. This leniency stems from the dictionary definition of self-interest containing an "or" and "especially". It matters not to me.

Then let's use the most atomic definition as provided by Dictionary.com without the "and" or the "or": regard for one's own interest. This is as close to a tautological definition as we can get and this suits my purposes just fine, as well as the purposes of the language community you are seeking to engage with.

Assuming all acts have an intentional aspect to separate them from regular physical motion, all human acts have at least some amount of "regard for one's own concern for a cause". Otherwise, I do not see how an act could take place without regard for one's own concern for the act itself taking place.

True enough, but the point should be made that you have not shown that the notion of interest is bound to "one's own concern" rather than "for a cause". If I have an interest in a cause then I do act with regard to my own interest in the cause, but if the cause is not related to my self-interest (that is, there is a discontinuity between my interest in it being a self-interest rather than an interest-for-other) then I am not acting out of self-interest because my interest in the cause is not related to self, but related to others through mediation of an interest-for-other.

All acts are out of self-interest (in a sense).

You have failed to distinguish between one's own self-interest in a cause and one's own interest-for-others in a cause. Ownership of an interest does not require it to be a self-interest (an interest directed towards self).

Just for fun though, I could try and tackle a more broad sense of self-interest by saying: desire only motivates our actions because it is a psychological state of stress. The same neural circuit in the brain responsible for fear is also responsible for desire, which makes sense because each concerns the world being in a state that one has been conditioned to think could be better.

When one acts based on a desire, one acts to alleviate some amount of psychological stress, otherwise the desire would not motivate one to act.

All acts are, to some extent, for the purpose of fulfilling a desire, and thus for the purpose of alleviating psychological stress.

All acts are out of self-interest.

This is more explicitly a version of psychological egoism and it suffers from the same pitfalls of circularity.

All men desire only freedom from psychological stress. Psychological stress from what? The psychological stress of not meeting their desires. Their desires for what? Their desires for freedom from psychological stress. Psychological stress from what? The psychological stress of not meeting their desires... ad infinitum... paraphrased from Joel Feinberg.