A novel being good is already a tall order to ask for, not to mention completed AND harem on top of that. But maybe Mushoku Tensei? Though the translation was pretty rough to say the least...
When it comes to harem, I would say 'Maburaho' is one of the best but it seems the translation was dropped so watch the anime if you want
I forgot the title but there's this Japanese novel about a guy who rented an apartment and then suddenly an odd bunch of girls started appearing one by one in that apartment, like a ghost, mahou shoujo etc,. There's an animation of it but only 1 season.
https://bakapervert.wordpress.com/magika-no-kenshi-to-shoukan-maou/ Other novels on that website is also harem. [I like Chinese harem novels more but almost none of the good shit is translated]
All Toika's novels: Everyone Else is a Returnee I Reincarnated For Nothing Stop, Friendly Fire! Infinite Competitive Dungeon Society
Mushoku Tensei has the best harem ever, the girls are introduced very early on but the romantic relationships took years to develop even if the friendship was strong from the beginning, the girls had different body type and personalities, they had their own objectives and their circumstances had actual weight on the story, every girl had her own perfect romance according to their personality, the harem is fully formed a little after halfway and the girls are still central characters with weight on the story to the very end, the complete opposite of the pokewaifu model Sevens is fine to see the MC being crushed by his harem, they are too many and too overbearing
FeralHeart , Arcane Emperor these r the only novels I know with a harem I find good. Experimental Log of the Crazy Lich doesn't have a harem perse it's more of group of girls chasing after the MC and the MC not being with anyone of them and when u read it u understand WHY. Same with Running Away from Hero ( Remake )