Strategic Asset Allocation for David and Sarah

David and Sarah are a couple in their early 40s with two children aged 10 and 12. They have a considerable risk appetite and are looking to establish a strategic asset allocation for their investment portfolio. Together, they expect to retire in 20 years and aim to maintain a lifestyle similar to their current one post-retirement. Their current investment portfolio totals $600,000, consisting of 60% equities and 40% fixed income. They are considering an increase in their equity allocation due to their time horizon, but they want to ensure they do not take on too much risk.

In addition, David is a finance executive earning $250,000 annually, while Sarah operates a freelance graphic design business generating approximately $80,000 annually. They are both contributing heavily to their 401(k) plans and are also saving for their children's education through a 529 plan. They seek advice on how to determine an optimal asset allocation that balances growth and risk. Discuss the strategic asset allocation process you would recommend for David and Sarah, considering their investment goals, risk tolerance, time horizon, and other relevant factors.