It might be different in other parts of the world, but this is the most common turtle where i come from.
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Note the claws. They're well over half an inch long, coated in noxious riverbottom slime, and the legs have a surprising range of motion. Yes, it CAN get you with its back claws if you grab the shell by both sides, ask me how i know. At least it's not a snapping turtle I suppose.
At the
very least, I'd require a PC picking up a turtle to have both hands free. Remember, in this scenario if you hurt the turtle (by picking it up carelessly, dropping it, etc) you risk it turning back to its original form, so caution is advised. Are you really going into the lich fight with empty hands, ready for turtle-grabbing duty? Cos if not, then you're having to drop your weapons/wands/whatever on the ground in order to free your hands to pick the thing up. Though I suppose you could choose to polymorph the critter into a carefully selected species of inoffensive miniature pet turtle with no claws that only eats grapes. Do they exist?
If you want a ridiculous metagamey combo, then one arcane trickster PC who can use bonus action mage hand to move a Bag of Holding next to the turtle, then another PC with the Telekinetic feat who can use their bonus action to shove the turtle in the bag then that'd get it done without requiring anyone to use their actions ... but that's probably getting well out of the scope of the original question.