A contract with thirty service levels is hard to manage, and one with none is hard to enforce. Five to eight usually cover what matters: invoices posted, payroll paid, the management pack, staff questions and compliance deadlines.
Each one needs a target you can measure, such as “within three working days of receipt”, and a place where you see the result every month without asking for it.
Agree what happens when a target is missed. A short written reason and a fix is the minimum; some contracts also reduce the fee after repeated misses.