The answer to this I would not have known until I was past 52 when my erections became less reliably ‘on demand’ than they had been. Thankfully that particular blood flow issue is easy rectified. Since I’m only covered by insurance for 8 per month, I cut them in half which works fine. Occasionally I don’t need them at all, but I rarely leave that to chance.
So yes I can cum without being fully erect. I’m not exactly soft either, because my thoughts (and my hand) are too busy for me to remain soft. And I’m not wasting a pill, even a half of one, on just rubbing one out.