If I understand correctly, your question is if you could masturbate (with a fleshlight) to deal with the urges without PMO. I couldn't talk about it from a scientific point of view but I could talk about it from my experiences. I've masturbated a few times (without porn) during longer streak and what I want to say is that I didn't like masturbating. I like the semen retention benefits, the benefits of keeping that energy inside me and not throwing it away with masturbation.
After a month on hard mode, I felt on top of the world. I had a lot of energy, motivation, low anxiety, no depression, I was overall more "alive" to the point where people (who knew me as a quiet, lethargic guy) asked me: "What's with you today? Did you drink too much coffee?" And I answered: "No, but I am full of life." Because I really felt full of life.
But then following a tough round of urges, I decided to masturbate without porn and this sent me into a flatline that made me feel the opposite of "full of life" from earlier. I told myself right there that I didn't like masturbating and then feeling like this. Plus, the whole day a chaser effect was killing me. I barely survived the day without masturbating again and it would've probably made me binge on masturbation, which wasn't what I had in mind. It also pushed me so close to checking some stimulating stuff online. The risk of going back to porn is not worth it. I think this is the number one reason why I wouldn't masturbate again: The fact that it could make me lose my mental balance and go back to binging on porn.
But anyway, this is my experience. I don't know what to say, maybe everyone should find their reason why they want or don't want to masturbate.