Personally, I think he's simply stuck and has no idea how to continue the series. No great plan, no conspiracy - he's simply unsure how to go forward, and I think the warning signs of this becoming a problem were visible as early as 2005. Maybe that's me speaking with the benefit of nearly a decade-and-a-half of hindsight (I first read the series in 2019, never watched the show), but I still stick to it.
A Feast For Crows and A Dance With Dragons broaden the scope of the story tremendously and don't move the plot forward all that much. So now there's got two books' worth of extra characters and plotlines for Martin to juggle on top of the extant plots and characters from the first three. GRRM seems to be looking for a way to neatly tie up everything within two books, when I doubt such a way realistically exists.
Hence his being stuck.