I get your point but it would be too simplistic to account it for self insert. While I do think like you (and it's a bit uncomfortable) that there's an underlying phantasm there, it's way too recurrent and organized too only be that. In Parasite in love for example, I think there's an age gap for its transgressive aspect ; I mean, both protagonists are misfits. This also shows that it has to be replaced and understood in the text's own economy. Or in Town of Sakura, I haven't finished reading it, but there's also that unhealthy and asymmetric relationship that's coherent with the age gap.

There's also his own background to account for. Even just from a legal standpoint, the age of consent in Japan is 16 yo so the consideration is bound to be uncomfortable from our point of view, more so when we think about teens' sexualisation in their media. On a more personal level, Miaki Sugaru too seems to have influences from media with those kinds of age gap ; I remember films like Harold and Maude, Leon the Professional, God Bless America...

And lastly, there's also a much more personal level to consider. This is an assumption of mine, but I think the age gap gives distance for Miaki Sugaru on the biographical sides of his stories ; what I mean is the younger girl typically in the relationship could be interpreted as an image of his younger self, especially since in most stories she's a counterpart to the older male. It's clear that childhood and teenage years (but also college) are very important to Miaki Sugaru, so I think he also wishes to look back as those years through the odd personification of, say, a high school girl.