Payroll was ran at the end of the year. IRS didn't deposit until the next year. Should the employers part of FICA be counted as a liability?

"They aren't written off until they are paid (2019),"

The employee share of taxes are part of their gross pay. The date you pay the employees is the date you incur the Total Expense. The Banking (payment to State and Feds) is not the date of Expense; it's the date of Banking.

The same is true of the employer's share: it is Debit Expense and Credit Liability, owed for the pay date. Not the Banking date.

The fact that you "wrote it off" as 2018 means you should already have the liability entered; otherwise, you cannot have the Gross Payroll entered. The math is:

Gross Expense is the Debit.

Credits go to bank (for takehome), tax liability, and perhaps the employer's expense or asset account as reimbursement, such as Health Insurance premiums or employee draw/advance repayments.

Then, later, when you reach the tax payment due date, you pay out Liability; it's not expense at this point.

If you "put it on" as a liability,  what are you planning on debiting (if not payroll taxes...) in order to keep the balance sheet *balanced*?

Let me address this statement: "I know that the bonus amount and the employees share of taxes are a liability on the balance sheet."

The Bonus amount already includes the employees' share of taxes. There is no separate amount for this; that's where it comes from. A Governmental Entity would accrue a Bonus as gross expense at year end, but then Reverse it for Jan, then pay it out as Expense in Jan. This avoids taking it as expense twice. You need to understand the accounting requirements for the entity you are managing, of course.