I played Gran Ende III to completion years ago and a tiny bit of I. I did not play II.
This is an RPG first and a porn game second or even third. It's a big, clearly ambitious game (let alone series) that a lot of work was put into. It's also not like the other games I played, which were fairly standard corruption games with above-average emphasis on story.
As a game, it's fine, and the main source of the 3 star rating. The story is interesting eventually - the twist was not what I was expecting. I don't think it's perfect but I can at least understand the general motivations of the baddies; I disagree but that's fine, and it's a hell of a lot better than a super generic "evil dark lord is gonna destroy the world because he's evil".
The gameplay is fine. It's an improvement over stock RPG Maker fare. You have multiple party members and thus specialization; instant attacks don't consume a turn and offer more flexibility, and there's a decent number of buffs and status effects available. The big thing is the stun system; bosses will charge up special attacks and you need to accumulate stun damage to block that move. This is something I dislike - not everyone can inflict stun, so it encourages the use of specific party members.
The other issue is the party size limit. You have 6 characters in total. You can use 3 at once. Having all 6 in a battle could make things into a slog, but the alternative is that you're facing some climatic battle to save the world... and then 3 girls are just hanging out in the back, watching. It really messes with the narrative. It also means that you'll probably end up getting used to a specific set of characters and just keep doing that, and let some people become perpetual back benchers, which isn't fair to them. That's certainly what happened for me. Sorry, Yunfa, I love your tits, but if there's a way to use you I can't figure it out.
Then there's the porn and romance. It is extremely tacked on. If all of the porn were cut out, you'd barely notice. I'm not going to give this a 2nd playthrough, but I strongly suspect there's no change in dialog if you're banging all of the girls or none of them. Affection points go up just from beating bosses, so you don't need to do night visits or sharing meals to raise them. All of the girls are fine, attractive, and agreeable, but that's it.
This is a harem game, but there's basically no side interactions, except at the very end. I haven't been following the thread closely, but it sounds like there will be some 3some scenes added later, at least. But as it stands now, the girls never sexually interact with one another. You can take a girl to dinner, take another to your room and pound her anus, then a third the next night, and no one ever comments on it. Sexuality never comes up outside of inns/home. It's not a harem so much as "parallel monogamy". In theory you could skip all of the porn and play it as a worksafe RPG, but why is that even an option? The romance and smut suffers as a result of it not being mandatory. Just make it a strict harem.