A milestone birthday party lives or dies by the drinks, and nobody wants to play host and bartender all night. When you hire a professional, you are paying for speed, skill, and someone else to deal with the ice and the cleanup. You want this booked as soon as you confirm the venue so you get the person you actually want instead of whoever is left.
Timing is everything here because good bartenders book out months in advance. We need to map out what to lock down early and how to spread the cost so you are not scrambling to pay the invoice the week of the party.
That 18-month window is a hard deadline, not free money, and any leftover balance when it ends will trigger standard interest charges. Pay off the total before the clock runs out so you are not paying extra for last year's birthday.