I understand that according the Magen Abraham if you don’t wear a Gartel אבנטa whole day you don’t have to wear one by prayers because of Hikon Likrath הכון לקראת but aren’t you supposed to at least close the buttons of your suit for respect to G-d for Hikoen הכון ?
Shalom, First of all let me apologize for the delay in answering your question. We have had some technical problems at the site, which have meant that I only now received your query. I have not seen any source that talks about buttoning one's jacket for prayer. The halacha is that one needs to dress as they would in front of important people, such as government ministers. It seems to me that many people wear open suit jackets even at very important meetings - and from a quick glance at news photographs, I see that even though many people do button their jackets in formal situations, many do not. This being so, my feeling is that if you personally always button your jacket when meeting those in important positions, it would be appropriate to do so when saying the Amidah. However, if you only sometimes button up, then you would not be obligated to do so during the Amidah, and it is fine to say the Amidah with an open jacket, as most people seem to do. Blessings