I don't think giving them a feature that says "Multiattack: Master Thief makes 3 attacks. Alternatively it can replace any number of those attacks with Cunning Action or Special Action." comes too long or cumbersome. Assuming I understood you right and that is what you are looking to do.
Legendary Actions, as has already been suggested is also a good call.
Or you can just give them 3 actions, that works. Because you are the DM you just have to notice and make a call when an unintended interaction comes up and avoid abusing it. Only thing I can think of is abilities where "as an action" is supposed to translate to "as their turn". Things like breaking out of an Entangle spell. You should probably avoid just doing that multiple times to not make the person who cast the spell feel like it was useless due to this unconventional situation.
The way they are currently set up the bottom shield is in front of the shield above it, so that when it gets pushed on it is supported by the next shield (which is again supported by the next shield, and so on).
It feels more intuitive that the shield should overlap the other way (instead of the top of your shield being supported by the next shield, the bottom of your shield would be supported by the previous shield) because the final shield in the row has to be the heaviest to hold and the first one is supported by the ground.