@CuRiSeeechun
I think he had one, but nothing to fill it with over a long time. Obviously there is already an "established" ending where some kind of romantic event happens (most likely harem marriage").
But the series got popular, and he likely didn't expect it to be that popular, like you said. It was going great for a while. But ever since MC went to the farm, it has been a mess. Even the hard set rules of the world were rewritten in order to make more plot lines possible, except for some reason MC is excluded from partaking in the new freedoms.
It also may be that the author just got bored of using the main girls, and is now just using them as background/transition characters for new plots. Ex: Lala. Supposedly a main girl, but has been completely pushed to the side by literally any other character. The inn keeper has had a larger impact on the overall story than she has.
If this is the case, I completely understand the author not making much new content for the series. Writer's block can be a death sentence for series if you try and force your way through it. Honestly, the best thing he can likely do is to end this series soon. I don't see how he could really revive the prior momentum it once had without it just getting hammered in the quality department. The best thing IMO is to revert the plot back to before they went to the farm and continue from there. Treat these 2 massive arcs as side stories, or ones that are "future events" that are down the road, or were seen in a dream or something. Basically make them not directly cannon.
I personally would enjoy exploring other girl's families (like Rachnee's spider family, not her prior host). Or maybe something like having them move to a newer house. Hell, he could even make a multi-part mini-arc where he reverses the roles of MC and each main girl in a 1 on 1 scenario. Whether that be MC is now a monster version of himself to the human version of {insert main girl}, or even go all in and make MC the monster girl and the respective girl a male human version of themselves (gender-bending + species swapping / body swapping).