Is it permissible to improve the form and wording of a da'wah (Islamic proselytizing) post—with the intention of influencing the reader and for the sake of Allah—or is that not permissible, as was understood from the action of one of the Salaf who burned the sermon he liked and proclaimed what he disliked?

There is no harm in improving the form and wording of a da'wah (invitation to Islam) post, if the work is done purely for the sake of Allah the Almighty. For Allah loves that when one of you does a deed, he does it proficiently and perfectly, because divine sustenance descends according to the work, and the more perfect and complete the work, the greater the rewards. What has been narrated from some of the Salaf is not considered a proof in this matter.