Lockdown database access on certain tables

I have a dev team who have a number of SQL databases used as a back end for mostly excel front end. This originally came from an excel spreadsheet until it got made into SQL databases. At first, they were not too concerned about security and decided to give access to a domain group logon that contains basically all the organisation's users. Over time they have put more sensitive information into the databases and have realised that anyone in the organisation can view the information if they were to use another tool, power BI or SSMS due to the domain logon. They say that they still need to use the domain logon as everyone needs access but wants to secure the sensitive information on certain tables but still be accessible to their queries from excel. They have suggested Signing Stored Procedures with a Certificate but I am concerned they will want to try this with all their code placed in SPs, removing the domain logon, and this would create numerous amounts of admin to maintain all of them. How am I able to implement this in SQL Server in another way?