How else could He do so?

it would be trivially easy for him to arrange a miracle which no other agent could replicate, and for that matter he could allow everybody to see it at once. This would remove all ambiguity and would be a much better way of achieving the goal of making his existence known, or further, of making his will known. If he does indeed use miracles to further his goals, it must be the case that he prioritizes hiding his goals and interventions almost as much as actually achieving them. (after all, nobody can come to an agreement about what God wants or what counts as a miracle) I can't think of a reason why this would be the case, so at this point I've abandoned the project of trying to justify it.

(a standard response would be that God's ambiguous interventions are a test that only the faithful will interpret as evidence. At that point he is actively discouraging the impartial weighing of evidence, which is another baffling behavior which would need motivation)