Here is a new idea that expands on the party mechanic. For the future only. As the MC romances certain harem members the romance tips for them could merge their name with the "name of the party" they are a part of that way it helps minimize the list. This could also lead to things that help romance the party as a whole during certain "relationship chat events." Also as an idea (for the future): When the reputation grows to a certain point maybe near the beginning ish you could have a "small ish party" that asks to join the guild to help get some ideas on how to implement it with other harem members in the future when they make their own parties if they want to according to the story you want to make. Especially with romancing that party all at the same time. Regardless of the mixed number of genders that might be in it (traps, futas, and girls). ;)
That is were the "lone wolf" idea can come in handy. If you get what I mean. Especially if you can only do so much with the mechanics that the engine lets you use. But do what you want to do. It always depends on the max amount of characters you want to use for the harem. A party system could develop a "party personality" amongst the characters in it. AKA romance the "(special named)" party. If that is a possible idea. If it makes it easier you could use the name (icon) of a certain party to do certain jobs the same way you have certain girls do certain jobs. Do what you gotta do at your own pace. Keep learning from your experience. Once you complete what you want done in this game. You could use that experience for the next game when you start from the beginning of it. Don't drop this one though. Please. You could also put some of these ideas on the back burner for the future once you get the story going really good. You could also use these ideas in a "minor way" as long as they don't over stress you. If it helps maybe you could make a name for different "parties" and then develop the love story line for those members as they come along. The main harem members will always be front and center. A big harem always has side harem characters that benefit the harem in their own way. Prioritize it like that. Especially for your first game. If you give each member equal attention then the story might not progress at the speed you may want. Once you get the hang of things "then" you could "spread the love."