Demon_King22
Nah remove the OG saber and rin. She's too dull and her character doesn't fit the mc since she's too righteous her alter form fit him better. While Rin is too naggy, I can't really see her in a harem with her character and she's also too weak as a master considering you already have manaka(didn't she summon 7 servant at once then alter them to have some fun? She's also really strong on her own). If you want a rin face it's better to use Erishkigal. She can pull her own weight and still relevant to the story with her strength. It's better to pick characters that can take care of themselves or have a good supporting powers. If not they will be easily forgotten and turn into damsel in distress most of the time.
You can just create a lost belt with him as the King and have characters he cultivated by his own efforts say like influence before reincarnation that he can have OC characters which he took interest upon with inspiration from Yokai's from other series but they must be different in character and power kind of like keeping it a prototype after that you can expand it as anew as you wish to do after all having characters from other series make it kind of a crossover or modelling it after Morgan would spoil the uniqueness.