Soft tub floor is a problem that several of us have experienced. You may be able to get easy access to the bottom of the tub by pulling the converter out of the wall. That's what I did. Then put the block on the floor under the soft spot. Make the block almost but not quite tall enough to touch the bottom of the tub, then squirt a lot of foam into the gap. Use a minimally-expanding foam.
Having gone through what you just did with the tub drain, you might want to mask off the area right around the drain, so you can do it all again if you need to.
Bill
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