Those fleshlights feel good because of their simplicity. A simple sleeve is less intense (so even STU while not looking like a low intensity sleeve is still less intense). A simple sleeve has no variety and allows you to get used to the sensation quickly during the action. Well before you are reaching a state in which you are about to cum. And at that point you can last a lot longer as you got a beyond that ‘surprise’ moment.
But 1 effect can matter: a low texture sleeve often compensates by tightness. And depending on the person tightness can have very diffirent effects on intensity. And usualy bigger girth gains less here as the tightness is just tightness. Bigger girth make the smaller details even less noticed then.
A sleeve i would recommend trying for that effect is heavenly, its very tight and has a lotus node. Its many changes also make the changes themselve less identifyable (which is more realistic compared to IRL). But as it has diffirent chambers of tightness, its not a flat sleeve. Its most likely not the weakest sleeve, but could very much act like it.
If heavenly is still too intense (but you like the lotus node), then obviously lotus would be the next one to go for, but that is a VERY weak sleeve as it has basicly no texture. But again, its compensated by using tightness instead. But i wouldnt start with this one. The chance its too weak is quite likely. You normaly dont want the situation in which full speed doesnt get you over the edge, idealy you want the sleeve to get you to this state automaticly.
But its a popular sleeve for a reason! What i stated is personal preferences (and on that, i prefer eden as ‘realistic’ sleeve, and alien as edging sleeve). Dont listen blindly to others, look at textures and just imagine how it would feel. Thats generaly the best indication to get. As those details are the one you are going to focus on while using it.