Considering the amount of capital you have available to invest, starting directly with multifamily properties could be a favorable option. With a monthly income of 30-50k, you may even have the opportunity to participate in multifamily syndications, gaining valuable insights from the inside.
If you prefer a more hands-on approach, starting with single-family properties can provide a solid foundation in real estate investing fundamentals. While there will be key differences in aspects such as financing and property management, the knowledge and experience gained from single-family investing can be valuable when transitioning to multifamily investments. Though the management and financing can be quite different on a multifamily property, the foundational knowledge/experience will be transferable.
Alternatively, starting with a property size between 4-10 units could offer a middle ground, allowing you to experience some benefits of both large multifamily and single-family investments. This size offers the opportunity to learn about and experience of managing multiple units, benefit from economies of scale, exploring financing options, and potentially considering partnerships.