Does not feeling at ease after Salatul-Istikharah and disliking the matter, despite repeating the prayer, mean that the prayer is invalid, or is it related to one's disposition, and must Salatul-Istikharah be repeated?

The Istikhara prayer is a Sunnah (recommended), not obligatory. Repeating it is permissible. It is not a condition for Istikhara that your aversion to the action disappears; rather, what matters is whether the action becomes easy for you or you turn away from it. We advise repeating the Istikhara prayer and seeking counsel, and not rushing to abandon a permissible and beneficial work, while keeping in mind its advantages and merits. If performing an obligatory duty properly becomes difficult due to aversion, it is permissible to leave it and seek another comfortable work. If there is no need for work, then staying at home is better.