What's the problem? The common AFV could still be amphibious in the MC version - short range ambphibious. Not high seas, not high speed, possibly no bilge pump.
Embarkation is simple, as is leaving such a sled. You simply drive over the edge. An empty sled can navigate even through the shallowest waters to the beach. The AFV would simply drive over the bow of the sledge if it doesn't make it with its load to the beach.
I'm sure that this is much more simple than a gold-plated-everything-in-one-beast option.
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