Caribdis has specifically said that the "harem" will form earlier in Eternum than it did in OiaLT, in part to allow time for more group sex content but also to show more of the interactions of LIs within the harem. There was this type of content in OiaLT but for several LIs it felt rushed and last-minute, imo.
Personally, I'd also like the MC to show a bit more agency and transparency in forming the harem. It kind of just falls in MC's lap in OiaLT where he's either caught in the act with other LIs that are just cool with it, or one of the other LIs does the "explaining" to a new LI off-camera rather than the MC actually telling them he's with other women.
Oh, and I also played Eternum before playing OiaLT. Going from the most recent scenes in 0.7 to the very first scenes in OiaLT was quite the jolt.