Start with the reason this software should exist, not your preferred technology. Which people will use this, with what frequency, and flutter developer hourly rate how is the job done today? An estimator who understands the goal can propose a cheaper route to it; a team that receives only the requirements as given will price exactly what you asked for.
Define what is included as user stories or scenarios: a walk through each important path. Equally important, list what is out of scope. An explicit list of exclusions removes more disagreement later than the rest or graphql of the brief combined. Also mark which parts are firm and which are still open — estimators price uncertainty, and pretending everything is fixed helps nobody.
Set out your constraints. The list covers the platforms and services involved, existing databases and their quality, compliance requirements, traffic expectations, supported browsers or devices and any technology you are committed to. If a deadline is real, say what depends on it: a good team is usually able to cut the right scope to hit it, laravel inertia vs livewire but only if they know it exists.
Say what completion means feature by feature. Clear acceptance criteria do not need formal language: a short paragraph stating what must be true when the feature works is enough. That one addition reduces the sign-off process dramatically and removes the usual argument at handover.
One last thing, say what you expect back. Require a task-level breakdown, the assumptions behind each number, whatever the team considers risky and a low number and a high number. Take a broad range as information, not evasion: it normally identifies the part of the brief that needs work. From there tighten that section and ask again — the revised figure tends to be much more reliable.