Why choose a turnkey system?
Turnkey systems are usually more expensive, they usually take a while to arrive, and there's still a shakedown period once you get them. So, why choose one? Because no matter how much of a tech genius you might be … you are not a team of engineers, software developers, or quality assurance engineers making sure your design works properly and reliably.
Let me share a brief anecdote. Although I've been developing and shooting videos professionally with multicopters for over a decade, there have been a lot of mishaps with custom aircraft. My shins are scarred from blade strikes; I've spent tens-of-thousands of dollars in crashed aircraft due to everything from engine failures to fly-aways. And I'm what you'd consider an expert in this field.
Building a reliable multicopter can be far less expensive if you make one that works properly out of the box. It can be a disaster (financially and otherwise) if it doesn't. It can't be rushed and you can't take shortcuts, and...