i feel like the author has forgotten about the time-travel setting and is just making a plain old rom-com at this point
Yeah, this basically happens with every isekai out there, however I think the point of the author doing this is often different than what you would expect and thus why they use it differently than what you expect:
Isekai basically allows the author to have a protagonist who is exactly like us within the world the author is introducing, so they can explain every facet of the world that should be known to the protagonist otherwise and make it seem natural (kinda pointless though given how generic these isekai worlds are). It can also allow the author write the protag to realistically make decisions that would otherwise seem insane for a native of that world.
Another thing it allows, which is also being done in this case, is to write a character that is knowledgeable and mature well beyond their years (think of how he handled the mother, or is acting in this case; granted he still acts immature and all the other kids are written to act fairly mature as well, but whatever).
So writing an origin like this allows the author to do a lot more subtle stuff that can greatly effect the story and character development, not just the more overt stuff like using their knowledge to get rich or constantly inventing things etc.
... but it could also be that they're just hack writers and forgot that this is a time leap story...