What do you do to prepare for scaling up?

We're a B2B engineering consultancy mostly selling hours and are currently gearing up to hire our first full timers beyond the founders, and boy is it scary.

We've identified a path to on-board and train them, and feel confident about that at least.

But how do you navigate taking on the extra workload so as to not overwhelm the existing staff, but also not having too little work with an extra salary to cover?

What systems should/can we prepare for quality management, book keeping, invoicing, quoting, etc? Our ideas currently seem either too heavy handed or too flimsy to scale with the 50-100% growth.

How much are we overthinking, and what are we forgetting?