Preparation can continue, but commitments are better held. Dependent TD steps depend on the principal's case, and if the degree route turns out to be the necessary one, retrieval of old academic records could extend the timeline meaningfully.
Separate what can be prepared from what should wait
Passports and civil documents such as marriage and birth certificates can be gathered now, since they are needed regardless of the route. A spouse and unmarried children under 21 may qualify for TD, through steps that differ by nationality, and TD permits study but does not authorize work of any kind. A spouse who intends to work needs an independent plan, which is worth starting now rather than after the principal's route is settled.
The principal likewise needs Canadian or Mexican nationality, a qualifying USMCA profession and credentials, prearranged employment, and an arrangement that is not US self-employment. TD permits accompanying the principal and studying; it does not permit employment. A spouse or child cannot cure a weak principal application, and a school plan does not create work permission. Plan the household budget on the principal's salary alone until each other person's classification is confirmed rather than assumed.
The Canadian principal separately selects the appropriate port-of-entry presentation or Form I-129 filing sequence.