Can I write off my entire property tax when I paid it in installments and the final balance paid was March 2019?

No, you can only take a deduction for the property taxes actually paid in 2018.

Only what you actually paid in 2018.  So not any 2019 payments.  Those will go on your 2019 return next year.

And be aware of SALT.  SALT is State And Local Tax.  Which includes property tax, any state tax paid like for last year’s return and includes any state withholding from your W2s and any 1099s you have.  You can only deduct up to 10,000 (5,000 MFS) for SALT State and Local Taxes.