The sacrifice is a cost. He leaves play in Step 4.
Compare his text to the text on King Robert's Warhammer, which works the way you suggest (and is arguably the clearer way to word an effect like this). When you make that comparison, it makes it seem that Roose's wording was probably deliberately chosen to make sure he does leave play, even if his effect is canceled.
The disconnect with referencing his current STR to determine X (which can't be done if he has already left play) is probably best resolved by remembering that you are going to have to determine X in Step 1 in order to check target eligibility and play restrictions. (For example, if Roose is the weakest character in play, you can't trigger the ability, even if you have a way to boost his STR before choosing targets.) The argument would be that for a triggered ability without a lasting effect (such as this one), X would be set, and remain unchanged, at this point in Step 1, and not re-determined or re-calculated when the targets are actually chosen in Step 5. (Similar to the way that once a cost of X is determined, the value of X in the triggered ability does not change if the cost is modified.)