The greatest miracle is indeed the greatest proof, that is the Quran.
This is repeated, but there is no proof of this. Mohammad (s) was a greater proof then the Quran, because he explained the Quran. Quran suffices as a proof but with Mohammad (s) and his family (a).
And one of the type of signs that manifest an aspect of Mohammad (s) was miracles he performed.
Now Quran says every people has a guide who such miracles and signs are performed by them and prove their position.
The only difference now, is that such miracles and signs are not in public, but the Mahdi is the beseeched and sought for such signs.
The Quran shows ALL the type of signs Quran talks about is shown to the certain. All types including physical miracles and inward light of Ahlulbayt (a) and Quranic type signs from words that are beyond humans.
You want to just emphasize on the "book" as signs from God, but the book itself emphasizes on all these type of signs.
So the Imam light is the highest type, connected to the doors to the sky reality type signs are the highest type of vision of certainty, but the Quran doesn't say this the only type of sign. There is words of Ahlulbayt (a) and Quran guidance, the book nature exalted above that of human capability, but the book itself emphasizes on physical miracles through out, not so we despair of being shown them, and not being told every people have a guide regarding such miracles and signs, to despair that the Guide of our time won't come to us and show us.
And showing miracles from the Guide can be a hard trial, true, many sorcerers seen his signs and accuse him of having power and knowledge he hides, and that he is hiding his supreme knowledge of sorcery from humanity. I've talked to people that have these ideas about Mohammad (S) the Prophet for example.
But all signs are a trial. The book we can make problems and interpret per our desires. Ahlulbayt (a) words we can show arrogance to. Physical miracles with arrogance, we might say it's sorcery and magic. Unseen signs, we are hallucinating and it's Jinn or Magic.
No type of sign is immune from disbelief. The day of judgment, however, 8 type of signs that the Quran talked about will be presented by his forces in a way no one will doubt and a higher level and most of the reason why people accept, is because, they are subdued. God doesn't let them talk except small whispers to each other. God taking full control and showing the truth and forcing it, and manifest all secrets, will happen, and people will know truth but too late.
But till then all type of signs shown can be denied. Yet Quran says all type of signs, not just one type (Quranic book words type) is shown to people who are certain.
The Guide in 13:7 is Imam Mahdi (a) in this age, and while hidden, there is still a guide to show such signs and miracles. Every people has such a guide.