Lookup Column Issue - List View Threshold exceeded

Customers List has Title field as Unique & Indexed Column.

Sale Invoices List has a lookup column (i.e., Customer) from the Customers List (Title).

My issue is:

Customers List has just crossed 5000 items. When I try to add a List Item in Sale Invoices List (where I have Lookup Column from Customers List), it throws an error as given below:

Error Message:- This is a lookup column that displays data from another list that currently exceeds the List View Threshold defined by the administrator (5000). To add items to the current list, please ask the list owner to remove this column.

I have increased temporarily the List View Threshold Limit to 6000 Items. It’s working fine now.

Is there any solution to fix the issue without increasing the List Items Threshold Limit?

Thanks in Advance.