Only talking 5 mins or so so need more time it sounds like. Also I try and keep hard hoping it will be even more intense. Practice makes perfect.
NO need to be hard at all, in fact as I might've written on this thread already, the difference between flaccid, semi, and erect penis stages can affect, enhance, allow, or even prevent prostate orgasms depending on the anatomy of the man using prostate stimulation toys with the hopes of achieving a hands-free dry prostate orgasm. Wet Os are great, but men usually get confused thinking "I'm not having a prostate O because I 'didn't end wet'" and that's not how to think about these kinds of orgasms.
Plus, there's the whole "rewiring" business of moving attention away from your cock and leaving it completely alone, hands off the whole time, while the toy moves inside your body. When you stimulate your cock, it jumps and flexes through pulsations and contractions that are autonomic nervous system related movements, i.e., you can't control the spasms they happen because of stimulus and its muscle reactions against the person's will. When those spasms happen, a man's anus contracts. When it contracts, it pulls the prostate massager into his body, and it contacts the prostate, which creates stimulation, which can trigger more automatic contractions. But, if the penis is involved in those movements, the body and mind of the man will connect penis stimulation with anal contractions, and prostate stimulation, and prostate orgasms might not, or will not, happen as a result. So to "rewire" the prostate and genitals, a guy has to take a no-hands-on-cock approach and let it just be what it is in whatever state it wants to be in, and let the stimulation be internal and anal/prostate focused and instigated instead.
I know how intense a rock hard cock can make prostate stimulation. I end every session with a full ejaculation, hard cock, with a toy in me so it fucks my ass as I'm cumming, and its a completely draining release orgasm. Nothing is stronger. But, I'll go the whole session until I just can't take it anymore and its like possession against my will to start stroking my cock and work up a massive eruption to end my session.
Try to not bring your cock into play when you do prostate play next time. Just let it chill. You'll likely wind up with some totally new sensations and probably full body orgasms as you shudder and shake with a completely flaccid penis!!