It’s extremely unlikely that RM’s built-in source templates can be changed. Doing so would be too disruptive to the RM user community. But you can copy any of the templates. After you have copied a template, you can change a field such as the repository from Master Source to Citation.
I have struggled philosophically with the concept of recording repositories. For example, back in the microfilm days of census data, I had three different public libraries and one Family History Center where I could find some of the same reels of census microfilm. It made me wonder how much difference the repository really made if I was getting the same micro-film no matter which repository I visited that day. Also, there were cases where if I were completely honest the repository for a book would have been something “Jerry’s living room, fourth book cabinet, third shelf, seventh book over”. That’s not a very useful citation to a repository, either. And moving online doesn’t change the quandary very much. Many documents are available from many different online sites.
I see your point and see where you are coming from. I have began adding the citation repository in the detail note field. That will work since the information is only for me anyway.
To my mind, and of course some will disagree, the Repository is where the original document is held. Often this is a local or national archive such as NARA or the National Archives in London. In UK the National Archives is where inter alia all the original census documents are held. I don’t really care how many people scan the documents and make them available online, as far as I am concerned the Repository is still the National Archives. My idea is that somebody down the line can go and look at the document and know where to find it regardless of whether Ancestry or FindMyPast still exist. Ancestry’s copy is no more than a scan. If I went to the archive and took a photo with my phone then I have a scan too. Does my Samsung phone become the Repository? I don’t think so.
It would never cross my mind to to cite a repository for a book, exactly for the reason that Jerry gives.
In all other cases, I name the repository as the place where the original document is held, not the online service that I viewed the digital image on - to my mind Ancestry (or any similar site) is never a repository