Shape Water creates shapes, not creatures
Let me challenge the frame of your question: you state you find it works best to run the animated shapes as creatures. But then, your entire question is about how it is not practical at all to run the animated shapes as creatures and how this creates problems for you and how that can be helped.
Don't try to work out how you can treat shapes as creatures. Do not treat them as creatures.
The shape is not a creature
The spell does not create a creature. It creates a shaped, moving volume of water. A few of the properties of that shape, such as that it can be moved and be shaped like a creature, are shared with a creature, but most are not: it has no hp, has no AC, has no speed, has no actions, it has no race or type, it has no ability scores. All of these lacking means you need to come up with them if you want to treat it as a creature.
Treating it as a creature creates more problems than it solves
Not only can your players easily create shapes in forms that have nothing to do with other creatures, like a cube, sphere, cylinder or whatever, in which case your approach to come up with equivalent creature statistics based on comparable creatures will fail. Also, even if they chose the shape of an existing creature, for most of the creatures you outline the game does not provide game statistics as they are harmless. This would force you to come up with them on your own as homebrew in a wide variety. It is just a lot of needless work and not practical for keeping the flow of the game going.
Treating it as a shape is easy
The problems all stem from trying to treat them as a creature. It is much simpler to accept that a shape of water does not interact with game elements that require a creature or game statistics: spells that target creatures will not affect it, damage will not harm it, conditions do not apply to it, it can perform no ability checks, it cannot attack.