You can’t control where the instrument change happens, I’m afraid. In this case Dorico really ought to wait until the end of the beam before it allows the instrument change, but at present it doesn’t consider beams when determining these transition points: it just looks at notes. A couple of approaches occur to me, though neither is great.
The first would be to make sure that the triangle part contains rests that match the position and duration of the rests in the other instrument’s part, whatever that might be: provided there’s a 16th rest at the same position in the triangle part as in the other part, then the bar will at least look correct, though the instrument transition point will still come before the end of the beam.
The second option would be to add a note in another voice at the same position as the 16th rest so that Dorico knows it shouldn’t create the transition there. That note could be scaled down to 1% in size or set to be transparent so it doesn’t appear.