Well, probably the best supported, and as a result, flexible, reference managers on the Mac just received a significant update.
I purchased it immediately, with a fair amount of relief. Relief since Bookends is easily the most frequently updated applications of all my Mac apps, and I had been wanting to pay again, as appreciation for all the work that is put in to both the Mac (and iOS companion) application.
If you are looking for a reference manager, across iOS and macOS, look no further. The others might be bigger, but how many of them frequently pose a question in their forums asking how many would support a user-request change, and then implement that change within weeks, if not days?
Bookends has been in existence since the very early days - and it shows, in a mature and powerful app that often is an example of "way more than what you expect under the hood"!