Okay, last three questions before I go and make the fic: How do you think Sabrina should get the Black Cat miraculous(right now, I'm thinking of either her passing Fu's test instead of Adrien, or the Cat being lost and ending up in Sabrina's possession somehow)? Who do you think should be the Ladybug(right now, I'm thinking either Marinette or Nino)? And finally, do you think Bee!Kagami should be in this, too, and if so, at what point?
Have Sabrina help Fu at the start instead of Adrien. Maybe she waited outside the school to make sure he arrives safely that first day. Adrien makes it into class just fine as a result, but Sabrina ends up late by helping Fu. Make it worse that by the time she shows up, instead of sitting behind Adrien as planned, Chloe just has Adrien sit next to her, bumping Sabrina out of her place beside her. Maybe she has Sabrina sit in front of her for easier copying. This would also allow Sabrina to be more affected by the interactions with and questioning from Roger.
I'd say keep Marinette and there's a reason why. Out of all the classmates, Marinette is the only one we see having any interactions with Sabrina as an individual and not just as an extension of Chloe. Having Sabrina and Marinette be the main duo would also lead to more to their dynamic with them being friends and partners as heroes but have some conflict as victim and follower of a bully as civilians. This setup would do more to emphasize how both of them work over the course of the story to get out of those roles, with both of them working to stand up for themselves and redefine who they are. Otherwise, no one else really has much to do with Sabrina in canon and you would need to try to add more to her interactions and relationships with the other classmates to make a similar dynamic work in a fic. Unless you have a specific idea in mind for Nino being the Ladybug and Sabrina's hero partner, I would start with limited initial changes and then let them snowball to best highlight how this one difference impacted the whole of the story.
If you want Bee!Kagami then go for it! Options include:
The Bee was another lost Miraculous and it has been in Kagami's family for generations. She may have been the Bee user from the start, and her coming to Paris was to retrieve the other Miraculous out of a sense of familial duty. Alternatively, it could be in her family's legacy, but she could be unaware of it until a later point when either her mother presents it to her in a crisis or she finds it by accident.
Go with canon and have Style Queen happen the same as canon. Only instead of Alya, say Kagami was the one to make it to the scene of the fight and Sabrina or Marinette can give the Bee to her. Alternatively, instead of Chloe, Kagami finds the Bee Miraculous afterwards and becomes the Bee user that way.
If you want Kagami as the Bee Hero sooner, you can make it a result of the changes Sabrina being the Black Cat hero have caused. Adrien not being Chat Noir means things would be different. Sabrina in his place means even more differences. Some akumas may not happen at all while others would occur instead. There could be a point when an akuma attack happens that the Bee would be better suited for and Kagami could be available. Such as say Frozer during her sorta date. Or Gorizilla if you change up events so she's the one involved in Adrien's chase across Paris instead of Marinette.
Or if you want something more original and less canon-based, say Kagami has a fencing rival who loses to her and gets akumatized. Have Kagami actually face the akuma as a civilian, maybe protecting Fu along the way. This could convince Fu that she would make a good hero and he gives her the Bee.
You have options!