My experience is with USN A-4s and classic F-18s. The subjective deceleration upon trapping is certainly robust, but not severe. Perhaps because you know exactly when it's coming. But it is very important to lock the harness prior to trapping. I too often learn the hard way and I only failed to lock the harness once. I flew before the introduction of the Joint Helmet Mounted Cuing System (JHMCS) which adds considerable weight to the helmet, so I don't know what that extra weight does to the experience. But looking at YouTube videos of modern Hornets trapping with the pilot wearing JHMCS (
LINK), it appears very similar to my memory of my experience. Don't know how much heavier the F-35's helmet system is, but I would venture the trapping sensation is very similar. I could not find a cockpit view video of an F-35 trap.