Tile – How much does a pallet of durock 1/4″ cement board weigh

You may break a few pieces(of marble) just picking them up.

A skim coat on either side of the joint will help.

Block feather with a concrete 'rubbing' block.

My marble floor always have Schluter Ditra under them. I also use it with any questionable floor.

Ceramic tile can tolerate L/360 deflection, stone is twice as sensitive at L/720.

If you have any doubts about floor springiness, use an isolation membrane or stiffen joists or add a beam. Cement board does little for stiffness.

Since they're up, I agree adding blocking is a PITA.

However, if its an inside wall that you have access to the other side of, use a "BIG GAP" polyurethane foam. Drill a series of 1/4" holes every 12" down the middle of each joist bay. Spray a 3 sec burst in each. Wait 12 hr and try the flex, it should be quite firm now.

You may not want to do this near the plumbing valve, for replacement purposes. It won't hurt it, though.