sweet torture

Honestly I can't see any progression.
Still keep saying "We're best friends" over and over

with them, you can't take what they say at face value. the author has given us plenty of moments where they showed more than tell us about their relationship. she also utilizes the side characters such as the mother and brother/fathers and how they view their relationship from the outside as well. again, this is one of those stories where the romance isn't super direct but it is very much an important factor in the story. and it's something that the main characters aren't fully aware of yet or are ready to admit to. so if you're looking for a in-your-face , fast pace, shounen ai, this ain't it lol. it's the opposite really. XD

No I'm not looking for "in your face" lol
I think they are both grown up and at least they can be honest with their own feeling. The artist could make at least some moments when one of them thinking "maybe... he's different?"
And artist mostly try to show the way people,parents or... see their relationship is some misunderstanding but in a cute, fluffy and funny way!

"Salad days" also is not a IN YOUR FACE type of shounen ai but you can feel and you can see they both have feeling.

That's right it's at the authors transgression to do whatever with them, and she prefers to go the more indirect route. She knows what she's doing, her side project webtoon has the main couple going at it like monkeys. Also the point of the side characters is not just for comical relief though, they're outside views into what everybody else can see of their growing relationship as the main couple can't see it themselves.
And 'Salad days' is wayyyyyy more upfront. The first chapter alone gave you no qualms that romance was going to be at the forefront of the story. 18 Unlimited has always been more subtle.

discretion* not transgression rofl.