In Professional Services, it is pretty typical to rebuild the main corporate website once every five years. The typical design process to "live" is often around 18 months with a considerable period of planning beforehand - gathering requirements and getting sign-off before an agency builds against the agreed specifications.
This process is horribly outmoded; it follows a "Waterfall" methodology, inherited from the construction industry, tracing its roots back to the 1970s. In construction, where changes to the design mid-project can be prohibitively expensive, you must analyze, design, spec, and document every aspect of your project at the start. Then the project steps through each stage to get to the bottom of the waterfall, with the final output being a physical building that exactly represents what was originally planned.
The reality of a modern digital presence is far more fluid than a building project, with new technologies and continually changing best practices. This makes the Waterfall approach the wrong methodology and even the idea of the website "rebuild" the wrong concept.
In modern technology projects built for demanding digital natives, the original specs survive only moments after they interact with the real world. This causes the plans to change as new opportunities and approaches are sought to be incorporated. This makes the journey down the waterfall increasingly chaotic; often culminating in a thunderous smash at the bottom. I am sure that this wasn't the analogy that they were looking for when the term was first coined but it is nonetheless true!
To counter this, technology companies in the early 2000s adopted 'agile' software development; it has since become the norm. Agile takes an iterative approach, prioritizing user feedback and swift 'minimum viable product' steps. This characterizes the website as a living, breathing entity that can grow and develop over time rather than a brick-and-mortar building that must be demolished to be modernized.
Taking this agile approach promotes a long-term partnership between your agency(ies) and the internal team. Instead of building once and walking away, the agency will build for the long term with issues like code quality and maintainability being given due weight. Quality of outcome becomes the benchmark, not website go-live dates or price per job.
Using a composable architecture is a key part of this agile approach. By using best-of-breed building blocks that can be added or removed depending on changing requirements, the website is more agile and more future-proof, the costs are clearer and the development process much shorter.
Regardless of which SaaS platforms you choose to build into your stack, to fulfill whichever marketing goal you are prioritizing, the aim is not to finish the web project per se. It is to create and maintain a truly class-leading web presence that is the pride of your colleagues and the envy of your competitors. One that continues to evolve to impress and engage your clients in the long run.