Yes you are correct, all power originates from Allah swt and no human being or creature can do anything without the power that Allah swt has given them.
About the imams (as), we say that they have a very high status with Allah swt; therefore if they make dua for someone, then there is more of a chance of our requests being accepted through them.
For example we can commit sins which block our Duas from being accepted, where as the imams (as) are of a status we cannot comprehend, and their duas are always answered. Just like if one in this life knows someone who is known to be pious. Yes, they can ask Allah swt themselves and make dua, but another way to get to Allah is by asking other believers to make dua for them as the supplications of the very pious people are more powerful. No one but a Jahil would claim this to be shirk.
This doesn’t negate the fact that a person should regularly and directly call on Allah swt especially through the Duas that the prophet and imams (as) taught us as they know how to best speak to Allah swt. Going through Ahlul Bayt (as) for one’s needs to be accepted is perfectly acceptable as long as one does so with the right aqeeda. One should know that rizq is only from Allah swt and the imams (as) do not give rizq but ask Allah swt for it. In this sense, they are the door of one’s needs.
It is also attributed to Imam Sadiq (as) that he said the following about 40:60 of the Quran:
1 heard Abu Abdullah (as) and he said:
‘Allah' does not Leave us to ourselves and had He Left us to ourselves, we would be like the ordinary people, and we are those Allah Mighty and Majestic Said: "Supplicate to Me, I will Answer you. [40:60)
Source: Basair al-Darajat
So while the verse is general and encourages one to make dua to Allah swt, it shows here how it is applied in the specific sense of the imams (as) being those whose supplications are answered, and there is no contradiction between the both explanations of the verse.
Therefore if one visits any of the shrines of the prophet or imams (as), they may ask them to make dua to Allah swt for them. This is not shirk as nawasib try to claim and they have no evidence to prove so.
For the sake of the argument let us say that we thought that the prophet and imams (as) could hear people at their shrines but in reality they couldn’t; would someone who spoke or asked them for something have committed shirk? No not at all. In the the worst case a person who spoke or asked them something would of just done a pointless action and not shirk (if we assumed the prophet and imams couldn't hear).
This isn’t case though as the prophet and imams (as) hear the words of the believers at their shrines and know about those who visit them.
May Allah grant you success