I think that something like this would be great in wiki form on MobileRead. The wiki area is available for editing, so if anyone wants to start adding this kind of info, feel free to put some structure together for it. I don't remember, but there might even be something similar started already. I've been wanting to put some of the info from this thread in the wiki also...
https://www.mobileread.com/forums/showthread.php?t=3968
I like the way you're thinking about what basic few pieces of info are most relevant for us to keep track of for each site. We want it to be useful info, but simple so it's easy to keep updated and to pick out with a quick read.
There are some sites trying to do some of this at a book-level. They whole "how do I find a good ebook to read in a format I can use" question is not very well solved at the moment. I think the high level approach you suggest swith some simple facts about various ebook source sites are organized in a simple way is a good idea.