Awesome! Most likely the best route would be to take your own lease on a space for a number of years. That way, you can design and arrange the space to suit your exact specification, company vibe and way of working, as well as amortise (spread) your upfront costs over the full term.
And when you move into a larger space, it’s usually more economical to manage the fit-out and utilities yourself than when you take a fully-managed flexible office.
That said, it depends on your rate of growth. If you’re growing so fast it’s hard to predict beyond the next year, you might be better off in a fully-managed flexible option. They can be pretty roomy these days… 400+ desks is no trouble. (Own floor, own building even, you get the gist.)