This is a really hard question for RMR imo!!
I think it goes for both of them that they are mutually more insecure (for very different reasons) at the beginning of their relationship, but also that once they're settled in it neither of them is generally shy to initiate. And they both like kissing.
I think the differences arise when it comes to PDA. That's limited while Mob is underage anyway (just Reigen's usual hands all over Mob, sure), but I think once Mob turns 18 he has a 'so what' attitude & would lose all care for kisses in public, which would definitely surprise Reigen, to say the least, because he on the other hand is much more worried about their obvious, visible age gap. Mob doesn't care. What are they gonna do? "You'll ruin my business, Mob."

Super easy for SasoDei because oral fixation boy can't keep his mouth to himself, ever. 🙄 Canon Sasori will not stop with the "useless, I can't feel that, it means nothing to me" talk (touch-starved liar) but regardless of AU, he will usually take long to reciprocate. Him initiating is almost always a surprise. Deidara feels like he won the lottery every single time.