If I understood correctly, the types you described kind like jailbait type. If that's the case, you probably need better relationships with mamasans so they could recommend you whenever very small freah meat are available. Because this kind new meat might not last long on the market. Sometimes it might last for a day only.
Find the right one to stick with is very tricky. You need all the right elements in place in order to make it happens. Pre-covid era, I have tried Taylor Swift type for less than a year, she was my type at the time, I wad madly addicted to her unfortunately wasn't the right one for me. The worst thing was I didn't even fuck her pussy from all the time and money I invested on her. We had some twilight love moments, I guess that's counted for something... Hell no.
Usually the second best thing could happen is find a bitch who actually like you instead of searching for the jailbait type but if you're lucky enough, when the moment is right, all the stars are aligned together. You might actually find both one day.
I once found the type maybe close to your descriptions, she's not very small (btw I also like very small type), pretty enough, definitely shy and vulnerable. I remembered Mamasan told me she found her from a supermarket counter so she was definitely inexperienced hence shy and vulnerable. Mamasan warned me she didn't take off clothes but the main selling points were her slim figure and pretty face so I was intrigued. Upon my first encounter, I wasn't disappointed at all. I was charming like our first blind date. I tried to feel her chest but she was shy and refused. By a slim moment of touth through her cloth, I could feel her extra peddle bra so I could feel she probably got not much down there and I was right later on. We ended up the session with both lied on the massage table, I was kissing her (probably evolved to dfk in that moment) and grabing her ass while she was fully clothed and jerked me off or I was fucking her hand or both. This great foundation helped me to make her my cum slut in the future. She even got a boob job "for me" (well she didn't tell me straight but I could really feel the huge difference).
So the bottom line is, when the opportunity represents itself. First you need to recognize and calculate the odds in your favor then developing the strategy depends on the situation and girl you encounter.