A little bit of an update.
I tried a bit of Mahler, namely the 2nd Symphony. I'm afraid to say that I didn't care for it. There didn't seem to be anything for me to latch onto. Maybe I started in the wrong place. Would there be a better introduction to Mahler? Perhaps moving away from the symphonies?
Last night I gave Sibelious 5th Symphony a go, and really liked it. Not as immediately impactful as Beethoven's 5th, and in many ways a calmer experience, but there is plenty to enjoy here. I've listened a couple of times, and I'll hopefully have time to give it another run through at some point today.
EDIT: Just to say that it was a second recording of the 5th that I enjoyed more than the first one I found. Obviously, I don't have enough knowledge to know why this was so, but there you go.