I don't think MP is a dreary novel - it is the ambience surrounding Fanny as evoked in the opening chapters which I find heavy-going. The change of atmosphere when the Crawfords appear struck me as interesting.

I'm in my mid-60s and have lost count of the number of times I have read MP - I agree very much that it improves with time. Not sure whether that is because my perspective changes as I get older. Maybe it is also that MP is such a densely written, multi-layered, thought provoking work which rewards re-reading and the sort of forensic examination we can do in a GR.