What should I do about my wife's neglect of the intimate relationship after childbirth, given my fear of falling short in my duties towards her if I marry another?

The husband's right to enjoy his wife is one of his most emphasized rights over her. He is permitted to enjoy her in any permissible way, and she is not allowed to refuse unless there is harm to her or it distracts her from an obligatory duty. The wife must fulfill this right without delay or aversion, and her obedience to her husband in this matter is obedience to Allah. If she refuses or fulfills the right with aversion, she exposes herself to the wrath of Allah. If the husband wishes to marry a second wife, there is no harm in it, provided he is capable of bearing the costs of marriage and maintaining justice between the two wives.