Please, no "they're too young, they're just children!" as justification for "take things slow". Yet in this same story other children are officially in a relationship. Why they couldn't be the MCs instead?
It's misguided to think that characters must first "understand themselves better" before entering a relationship, as if there wouldn't be even more (and more interesting) things to learn after.
I'll continue reading this, of course, but it's a small disappointment that author failed to surprised me and this will be just another "standard japanese romance" (and the way characters interact don't even change that much and they get together anyway).
You're using "standard Japanese romance" like a pejorative and that's just not really correct, though.
There are more-successful examples of romance manga, and less; I would argue this falls into the former, and that it's also a
slowburn romance manga, and thus the pacing will reflect that. You can not personally enjoy that sort of story, and that's fine—but you're making a lot of objective statements based on subjective preferences and that's not really going to sway anyone to your side of things. Calling the author a coward and acting like Shunpei not confessing outright is categorically poor writing (which is what you appear to insinuate with the "standard Japanese romance" line) is your own opinion. You're entitled to it, but it's not gospel truth and doesn't make this a bad story in the general sense.
Also, An and Shunpei's dynamic are different from the two friends who are currently dating; those two have been in the same class and school the whole way through and were shown to have their feelings for one another develop over time, not all at once. An and Shunpei, by contrast, were separated once middle school started, and
didn't have a foundation of feelings to build upon prior to that point: they're starting from zero, with limited time spent around one another, and they weren't romantically inclined from the start. This is a story of them opening their eyes to that world,
and to the fact that they are doing that
for one another.
Like, this isn't even a "bad romance portrayal"; they are consistently progressing forward since the point they reconnected with one another, and there's no repeated moments of backsliding where they run away from one another for multiple chapters—and when it happens or starts to happen, one of them reaches out to the other and addresses the issue and they progress together again. It happened between last chapter and this one, when Shunpei said he wanted to talk at the shrine instead of letting An just get off the bus immediately following the "Shunpei's secret girlfriend" rumor.
Yes, it's taking its time, but that's not a bad thing. Whether you enjoy it is your call to make, but your being disappointed isn't the author doing a bad job at the story they're trying to tell.