If 98% of your customers have short-ish bills and are fine and 2% have long bills and have performance issues, maybe the simplest answer is to do some optimisation specifically for the 2% customers.

You could look at creating a table or tables optimised for long bills (or maybe an indexed view...) and look how you get the performance down for this group of customers.

It is then a fairly easy job to have an overnight or weekly procedure that finds the Customer Ids for the problem customers and puts them into a lookup table. Then add some code that uses the small or large customer logic depending on what is found in the lookup table.

Regarding hoping that Microsoft will make it easy to use a writeable hub with read-only spokes for scale-out, I don't think it will happen.