What did Robert Browning mean with "here were the end, had anything an end"

Browning's use of the subjunctive tense can make this line a little bit confusing. The line comes from the beginning of the final main section of his long poem The Ring and the Book. Its parallel can be found in the note that "now decline must be." What Browning is saying in this line is that, if anything can be said to have an end, this is where the end of this particular story lies. More colloquially, he is saying: here's the end of the story, if anything really does "end." The implication is that there is no real conclusion to anything except in the place where we, as the narrators of tales, decide to stop telling them.
Specifically, Browning is here announcing the start of the concluding act, as it were. He depicts the decline of the story, and also the decline of his protagonist, in this section. Browning is announcing not only what will happen to Guido, but also that the end of the story is coming up and that this section will bring it to a close.