Filling the gap between deck boards
I'd consider how much rain it gets before thinking of filling the gaps. You could be swapping one problem for another if the deck doesn't slope sufficiently away from the house to properly drain.
My first plan would be
A leaf blower (or even just a yard brush) for the dry stuff, followed by
A pressure washer for what's left. A bit of decking cleaner in the soap container every so often would preserve its appearance too.