This question raises some flags for me. If our product architecture is being described as 'monolith' yet the business is at a stage where we expect rapid growth and need to define a strategy to scale, then we probably are already behind the 8 ball a bit and it's going to be a game of hard choices. But, as you would in any such situation, you rely on tested principals of design and make strategic gains where possible by attempting to keep elements of the solution being worked on modular, modern (future-proof), and free of tech debt. It's hard to say more specifically without a lot more detail of course, but this should start with a clear mission statement from leadership that is translated into an achievable roadmap for engineering and is clearly articulated by the product architecture team so that all stakeholders understand where the wins and losses will be, and everyone buys in to the tradeoffs that inevitably must be made to achieve the objective. Once this is done all that's left is simple execution and risk management.