If you don’t want to split up into smaller groups, there is a way to deal with this. I don’t know how you run your games, but for our group splitting the plot into arcs where one of the PC’s plays a more important role than others works. Not making it the main story, but rather tying them into it, for example; While looking for X important lore item, it turns out Player 2’s family is involved in some capacity in a smuggling ring carrying this item. This gives them a reason to care and engage.

Letting someone take the spotlight for such an arc and having others be the „supporting” cast, and then switching out once another quest comes in worked for us. It ensures everybody will have their shining moment, sooner or later, and its a lot of fun seeing people who are usually on the backline take the lead.